Method of manufacture of advanced heterojunction transistor and transistor laser

ABSTRACT

Methods of manufacture of advanced heterojunction transistors and transistor lasers, and their related structures, are described herein. Other embodiments are also disclosed herein.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Patent ApplicationNo. 61/800,175 filed Mar. 15, 2013 and to U.S. Provisional PatentApplication No. 61/885,434 filed Oct. 1, 2013, the contents of both ofwhich are hereby incorporated by reference.

TECHNICAL FIELD

This description relates generally to semiconductor devices, and moreparticularly to advanced heterojunction transistors and transistorlasers.

BACKGROUND

Heterojunction transistors, including heterojunction bipolar transistors(HBTs), have been used for many decades. For example, HBTs using silicongermanium (SiGe) as the base layer for the HBTs and using silicon (Si)as the emitter layer for the HBTs have been used for a variety ofapplications, as have HBTs using gallium arsenide (GaAs) as the baselayer and aluminum gallium arsenide (AlGaAs) as the emitter layer.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a general configuration of a bipolar transistor as a threeterminal device in its two constituent forms NPN and PNP.

FIG. 2 shows a general cross-sectional device depiction of a bipolartransistor in a vertical stack geometry.

FIG. 3 shows a flatband energy diagram for three typical heterojunctionsituations between the emitter and base materials: type I, type II, nearzero conduction band offset.

FIG. 4 shows a flatband energy diagram for three typical heterojunctionsituations between the base and collector materials: type I, type II,near zero conduction band offset.

FIG. 5 shows a flatband band edge diagram through an NPN transistor foroptimized transport.

FIG. 6 shows a flatband edge diagram through a PNP transistor foroptimized transport.

FIG. 7 shows a general configuration of an injection laser diode, with aquantum well or quantum dot active region.

FIG. 8 shows a flat band energy diagram of a general configuration of aseparate confinement heterostructure laser with an active QW or QDregion which can be of the type I heterojunction band alignment.

FIG. 9 shows an example of a cut away device depiction of an edgeemitting injection diode laser.

FIG. 10 shows an example of a cross-sectional device depiction of aVCSEL, where light is coming out of the bottom, but could be designed sothat light comes out of the top.

FIG. 11 shows a simple diagram of an NPN transistor laser showing thethree terminal device configuration with a corresponding free carriertype designation, with a quantum well or quantum dot active region inthe base.

FIG. 12 shows a possible cross-sectional device depiction of an NPN edgeemitting transistor laser or light emitting structure.

FIG. 13 shows an example of a possible cross-sectional device depictionof an NPN VCSEL transistor laser where light is coming out of the top,but also could be designed so that light comes out of the bottom.

FIG. 14 shows energy band structure diagrams for Ge and Sn semiconductormaterials.

FIG. 15 shows a graph of collector current density J_(C) vs. turn-onvoltages (V_(BE)) of various HBT material systems.

FIG. 16 shows the hole concentration of GeSn films as a function of Sn%, as measured by Hall effect.

FIG. 17 shows a representative graph showing a GeSn direct band gapenergy vs. its lattice constant.

FIG. 18 shows a range of wavelengths that can be achieved by direct gapGeSn.

FIG. 19 shows a formation of quantum dot structures resulting fromself-assembled GeSn quantum dots by the Stranski-Krastanov (SK) methodthat transitions from two dimensional planar growth to island growth.

FIG. 20 shows a flat band energy diagram of a GeSn quantum dot to Siwhich can be of a type II heterojunction band alignment.

FIG. 21 shows a possible range of emission wavelengths that areachievable in a type II GeSn quantum dot heterostructure with Sibarriers. This possible data is for low Sn % GeSn alloys.

FIG. 22 shows a flat band energy diagram of GeSn quantum dot with SiGebarriers which can be of type I alignment.

FIG. 23 shows a methodology of planar growth of a QW region on top of abarrier layer, and then growth of another barrier layer on top of the QWlayer.

FIG. 24 shows a flat band energy band type I alignment of a GeSn QW witha GaAs barrier.

FIG. 25 shows a general flat band energy diagram of an NPN GeSn doubleheterojunction bipolar transistor.

FIG. 26 shows a flatband energy diagram that shows an NPN HBT with acompositionally graded GeSn base (Ge or low Sn % graded to higher Sn %GeSn).

FIG. 27 shows a resulting flatband energy diagram of a GeSn quantum wellor quantum dot material placed in the base region of a heterojunctiontransistor.

FIG. 28 shows where a flat band energy diagram of a GeSn quantum well orquantum dot where a barrier has been graded from a base material to aGeSn active region.

FIG. 29 illustrates an exemplary flat band energy diagram of an NPNstructure of a GaAs Emitter-GeSn Base-GaAs Collector symmetric doubleHBT.

FIG. 30 shows an exemplary cross-sectional device depiction embodimentof an NPN GaAs—GeSn—GaAs symmetric double HBT.

FIG. 31 illustrates an exemplary flat band energy diagram of an NPNstructure of a GaAs Emitter-compositionally graded GeSn Base-GaAsCollector double heterojunction transistor where the base is graded GeSn(Ge or low Sn % graded to higher Sn % GeSn).

FIG. 32 shows an exemplary flatband energy diagram of an NPN transistorlaser structure with a GeSn QW or QD active region in a Ge P-typebase/barrier material 3200.

FIG. 33 shows a possible cross-sectional device depiction of an NPNtransistor laser structure with a GeSn QW or QD active region in a GeP-type base/barrier material 3300.

FIG. 34 shows an exemplary flatband energy diagram of an NPN transistorlaser structure with a GeSn QW or QD active region in a GaAs P-typebase/barrier material 3400.

FIG. 35 shows a possible cross sectional device depiction of an NPN edgeemitting transistor laser structure with a GeSn QW or QD active regionin a GaAs P-type base/barrier material 3500.

FIG. 36 shows an exemplary flat band energy diagram of a SCH laserutilizing a GeSn QW or QD region located in UID Ge barrier/waveguideregion with GaAs cladding.

FIG. 37 shows a cross-sectional device depiction of a SCH laserutilizing a GeSn QW or QD region located in UID Ge barrier/OCL regionwith GaAs cladding.

FIG. 38 shows an exemplary flat band energy diagram of an SCH diodelaser utilizing a GeSn QW or QD region located in UID GaAsbarrier/waveguide region with type InGaP cladding.

FIG. 39 shows a planar growth of strained GeSn (low Sn %) on Ge, with Gebarriers above and below a QW GeSn film.

FIG. 40 shows an island growth of strained GeSn (high Sn %) on Gebarrier layer with a subsequent formation of a QD layer.

FIG. 41 shows an exemplary flat band energy diagram of an NPN HBT withan ordered InGaP emitter, GeSn base, GaAs collector.

FIG. 42 shows an exemplary flat band energy diagram of an NPN HBT withan ordered InGaP N emitter, a graded GeSn (Ge or low Sn % graded tohigher Sn % GeSn) base and a GaAs N collector.

FIG. 43 shows an exemplary flat band energy diagram of an inverted NPNHBT structure where an emitter is grown first, and a base material istensile strained Ge, collector up structure.

FIG. 44 shows an exemplary flat band energy diagram of an inverted NPNHBT structure where an emitter is grown first, and a base material istensile strained GeSn, collector up structure.

FIG. 45 shows an exemplary flatband energy diagram showing in an NPNconfiguration a compressively strained GeSn HBT collector grown first,emitter up.

FIG. 46 shows an exemplary flatband energy diagram showing a GeSn DoubleHBT Structure graded Emitter and graded Collector grown first, where aGeSn base may or may not be compressively strained.

FIG. 47 shows an exemplary flatband energy diagram of an NPN AlGaAsEmitter-GeSn Base-GaAs Collector double HBT.

FIG. 48 shows an exemplary flat band energy diagram of an NPN HBT laserwith AlGaAs emitter/cladding and AlGaAs collector/cladding.

FIG. 49 shows a possible exemplary flat band energy band diagram for asymmetric double heterojunction GeSiSn emitter-GeSn base-GeSiSncollector structure which can work as an NPN or PNP transistor device.

FIG. 50 shows a possible exemplary flat band energy band diagram forGeSiSn emitter-graded GeSn (Ge or low Sn % graded to higher Sn % GeSn)base-GeSiSn collector structure double HBT 5000 which can work as an NPNor PNP transistor device.

FIG. 51 shows a possible exemplary flat band energy band diagram for asymmetric double HBT where a GeSn QW or QD is embedded in a GeSiSn baseregion with SiGe emitter/cladding and SiGe collector/cladding.

FIG. 52 shows an exemplary flat band diagram of a Si Emitter-SiGe basewith GeSn QD-Si Collector light emitting HBT.

FIG. 53 shows a possible depiction of a cross-sectional device depictionof a Si based edge emitting transistor laser or light emittingstructure.

FIG. 54 shows an additional of the variation laser structure of FIG. 52,using Si_(0.6)Ge_(0.4) as a Base/Barrier material.

FIG. 55 shows an exemplary flat band energy diagram of an SCH laserutilizing a GeSn QW or QD region located in UIDGe_(1-x)(Si_(0.8)Sn_(0.2))_(x) barrier/OCL.

FIG. 56 shows energy band gaps of various semiconductors vs. theirlattice constant.

FIG. 57 shows an exemplary wafer bonding process that enables monolithicjoining of two dissimilar semiconductor materials.

FIG. 58 shows an exemplary flat band energy diagram wafer bonded NPNGaAs—GeSn—GaN HBT.

FIG. 59 shows an exemplary flat band energy diagram of an NPNGaAs-graded GeSn—GaN HBT.

FIG. 60 shows an exemplary cross-sectional device depiction of a waferbonded GaAs—GeSn—GaN NPN double HBT in a mesa configuration.

FIG. 61 shows a possible exemplary cross-section embodiment of a waferbonded GaAs—GeSn—GaN NPN double HBT in a mesa configuration.

FIG. 62 shows QuantTera's wafer bonder configuration.

FIG. 63 shows a current-voltage characteristic of a wafer bonded P GeSnto N⁻ GaN showing PN rectifying behavior.

FIG. 64 shows an exemplary flat band energy band diagram of an NPN InGaPEmitter-GeSn Base-GaN Collector Double HBT.

FIG. 65 shows an exemplary flat band energy diagram of an NPNInGaP-graded Ge—GeSn—GaN HBT.

FIG. 66 shows a schematic methodology of the epitaxial lift off process.

FIG. 67 a shows a schematic of the top half of an HBT InGaP emitter/GeSnbase stack with the inclusion of an AIAs sacrificial layer.

FIG. 68 shows a pre-processed top half of an HBT Top.

FIG. 69 shows where a top half of an HBT wafer is bonded to GaNcollector structure.

FIG. 70 shows an inverted top half of an HBT.

FIG. 71 shows the straightforward wafer bonding of an inverted top halfof an HBT to a GaN collector structure.

FIG. 72 shows a cross-sectional device depiction of a fully wafer bondedHBT structure.

For simplicity and clarity of illustration, the drawing figuresillustrate the general manner of construction, and descriptions anddetails of well-known features and techniques may be omitted to avoidunnecessarily obscuring the present disclosure. Additionally, elementsin the drawing figures are not necessarily drawn to scale. For example,the dimensions of some of the elements in the figures may be exaggeratedrelative to other elements to help improve understanding of embodimentsof the present disclosure. The same reference numerals in differentfigures denote the same elements.

The terms “first,” “second,” “third,” “fourth,” and the like in thedescription and in the claims, if any, are used for distinguishingbetween similar elements and not necessarily for describing a particularhierarchical, sequential, or chronological order. It is to be understoodthat the terms so used are interchangeable under appropriatecircumstances such that the embodiments described herein are, forexample, capable of operation in sequences other than those illustratedor otherwise described herein. Furthermore, the terms “include,” and“have,” and any variations thereof, are intended to cover anon-exclusive inclusion, such that a process, method, system, article,device, or apparatus that comprises a list of elements is notnecessarily limited to those elements, but may include other elementsnot expressly listed or inherent to such process, method, system,article, device, or apparatus.

The terms “left,” “right,” “front,” “back,” “top,” “bottom,” “over,”“under,” and the like in the description and in the claims, if any, areused for descriptive purposes and not necessarily for describingpermanent relative positions. It is to be understood that the terms soused are interchangeable under appropriate circumstances such that theembodiments described herein are, for example, capable of operation inother orientations than those illustrated or otherwise described herein.

The terms “couple,” “coupled,” “couples,” “coupling,” and the likeshould be broadly understood and refer to connecting two or moreelements or signals, electrically, mechanically or otherwise. Two ormore electrical elements may be electrically coupled, but notmechanically or otherwise coupled; two or more mechanical elements maybe mechanically coupled, but not electrically or otherwise coupled; twoor more electrical elements may be mechanically coupled, but notelectrically or otherwise coupled. Coupling (whether mechanical,electrical, or otherwise) may be for any length of time, e.g., permanentor semi-permanent or only for an instant.

“Electrical coupling” and the like should be broadly understood andinclude coupling involving any electrical signal, whether a powersignal, a data signal, and/or other types or combinations of electricalsignals. “Mechanical coupling” and the like should be broadly understoodand include mechanical coupling of all types. The absence of the word“removably,” “removable,” and the like near the word “coupled,” and thelike does not mean that the coupling, etc. in question is or is notremovable.

DETAILED DESCRIPTION

The fabrication of a germanium tin (GeSn) based heterojunction bipolartransistor and/or light emitting transistor or transistor laser or lightemitting device or laser for electronics and photonics is describedherein. Where GeSn is used as the base material in a heterojunctiontransistor, the GeSn, GeSn quantum dot, GeSn quantum wire, and/or GeSnquantum dot material can be used as the active region of a lightemitting transistor or transistor laser or light emitting device orlaser. In one embodiment, a heterojunction bipolar transistor includes aGeSn base region. In another embodiment, a method of manufacturing aheterojunction bipolar transistor includes forming a GeSn base region.In a further embodiment, a device includes: a first heterojunctionbipolar transistor comprising a PNP device having a first GeSn base; anda second heterojunction bipolar transistor comprising an NPN devicehaving a second GeSn base, wherein the first and second heterojunctionbipolar transistors are located over a common substrate. In anotherembodiment, method of manufacturing a device includes: forming a firstheterojunction bipolar transistor comprising a PNP device having a firstGeSn base; and forming a second heterojunction bipolar transistorcomprising an NPN device having a second GeSn base, wherein forming thefirst and second heterojunction bipolar transistors occur simultaneouslywith each other over a common substrate. In yet another embodiment, adevice includes: a first bipolar transistor comprising a first GeSnbase; and a second bipolar transistor comprising a second GeSn base,wherein the first and second bipolar transistors are complementarydevices and are located over a common substrate. In a furtherembodiment, a method of manufacturing a device includes: forming a firstbipolar transistor comprising a first GeSn base; and forming a secondbipolar transistor comprising a second GeSn base, wherein forming thefirst and second bipolar transistors occur simultaneously with eachother over a common substrate. In still another embodiment, a bipolartransistor includes a GeSn base region, and in yet another embodiment, amethod of manufacturing a bipolar transistor includes providing a GeSnbase region. In a further embodiment, a transistor laser includes a GeSnactive region, and in another embodiment, a method of forming atransistor laser includes forming a GeSn active region. The descriptionherein elucidates a methodology for making a heterojunction bipolartransistor (HBT) that utilizes GeSn as the base material. Furthermore,the unique properties of GeSn can be utilized as the active region of avariation of the transistor which is the transistor laser, or in a lightemitting device like a laser. Embodiments described herein can relate tothe following: GeSn which has the smallest band gap energy for thematerial systems GaN, GaAs, Si, InP, Ge, Sn, AIAs, InAs, GaP and thuswould be useful for making a heterojunction bipolar transistor, laser ortransistor laser device. Nomenclature: Ga (gallium), N (nitrogen ornitride), As (arsenic or arsenide), Si (silicon), In (indium), P(phosphorous or phosphide), Ge (germanium), Al (aluminum), Sn (tin), Sb(antimony or antimonide), B (boron), C (carbon, carbide). Theembodiments can relate to the following:

-   -   1) Bipolar transistor using a GeSn base    -   2) Bipolar transistor using a graded Ge—GeSn base    -   3) Bipolar light emitting transistor or transistor laser using        in the base region a GeSn quantum well, quantum wire, or quantum        dot active region.    -   4) Light emitting or laser structure using GeSn quantum well,        quantum wire, or quantum dot active region.        The same or different embodiments can relate to:    -   1) Lateral structures (edge emitting)    -   2) vertical structures    -   3) inverted vertical structures    -   4) Indirect band gap GeSn    -   5) Direct band gap GeSn    -   6) Multiple quantum well layers    -   7) Multiple quantum dot layers    -   8) Multiple quantum wire layers

In one embodiment, a heterojunction bipolar transistor can include aGeSn base region. In another embodiment, a method of manufacturing aheterojunction bipolar transistor can include forming a GeSn baseregion. In a further embodiment, a device can include a firstheterojunction bipolar transistor comprising a PNP device having a firstGeSn base, and a second heterojunction bipolar transistor including anNPN device having a second GeSn base.

A bipolar transistor or bipolar junction transistor is a three terminalor three layer device that relies on doping (adding “impurity” atoms) ofthe semiconductor layers to form N-type or “N” (electron surplus layer)semiconductor and P-type or “P” (electron deficient layer) semiconductorto form PN junction (diodes) in a three terminal configuration. Thisthree terminal or three layer device can include back-to-back PNjunctions to form a three layer sandwich with each of the layers namedthe emitter, base, and collector. There are two kinds of bipolartransistors, NPN and PNP.

FIG. 1 shows the general configuration of the bipolar transistor as athree terminal device in its two constituent forms NPN 0101 and PNP0108. The NPN 0101 structure comprises an N-type emitter 0102, connectedto a P-type base 0103, which is then connected to a N-type collector0104 region, which comprises the three terminal device. Thecorresponding currents in the three terminal device correspond to theemitter current I_(E) 0105, base current I_(B) 0106, and collectorcurrent I_(C) 0107. The NPN 0101 device has a junction at theemitter-base, where the applied voltage is V_(BE) 0115, and a secondjunction at the base-collector, where the applied voltage is V_(BC)0116.

The PNP 0108 structure comprises a P-type emitter 0109, connected to anN-type base 0110, which is then connected to a P-type collector 0111region, which comprises the three terminal device. The correspondingcurrents in the three terminal device correspond to the emitter currentI_(E) 0112, base current I_(B) 0113, and collector current I_(C) 0114.The PNP 0108 device has a junction at the emitter-base, where theapplied voltage is V_(BE) 0117, and a second junction is at thebase-collector, where the applied voltage is V_(BC) 0118. Thebase-emitter voltage V_(BE) turns on the transistor and generally isoperated in forward bias, and the base-collector voltage V_(BC) isgenerally reversed biased and also determines the breakdown voltage ofthe device.

The base region controls the operation of the transistor. Thecharacteristics and properties of the base material and the base-emitterjunction and the base-collector junction are the dominant factors thatdetermine the electronic properties of bipolar and heterojunctionbipolar transistors. Thus the utilization of a new type of base materialin these transistors allows for the development of vastly improvedtransistors for high speed and power efficient operation.

Semiconductors can be discussed in terms of their energy band structure.The energy band structure shows the allowable carrier (electron or hole)energy states for semiconductor as a function of the crystal momentumdirection. The energy band structure can be divided into two mainregions: the conduction band; and the valence band. N-type materialconduction relies on free movement of electrons in the conduction bandof the material. The conduction band can be characterized by theconduction band energy level (lowest energy in the conduction band).P-type material conduction relies on the free movement of holes (hole:absence of an electron) in the valence band of the material. The valenceband is characterized by the valence band energy level (or the highestenergy level in the valence band). The difference between the conductionband energy level and the valence band energy level determine the energyband gap of the semiconductor (difference of the conduction band energyminima to the valence band energy maxima).

At the PN junctions of the bipolar transistor, there exists a depletionzone that in the absence of an externally applied electric fieldprevents the movement of the charge carriers across the junctions ordifferent layers. The operation of this device relies on two types ofcarriers, free electrons (negative charges in the conduction band) andfree holes (absent electron charge carrier, positive charge in thevalence band). Thus, the name bipolar is ascribed to this device becauseits operation involves both electrons and holes, as opposed to unipolardevices like field effect transistors whose operation involves only oneof electrons or holes.

The bipolar transistor has three distinct regions: the emitter, thebase, and the collector. The flow of charges (called electrical currentor current) in this transistor is due to the bidirectional diffusion ofcharge carriers across the junction. The bipolar transistor is biased asfollows. The emitter is forward biased via the contact pads with thevoltage potential (base-emitter voltage V_(BE)) to force charge carriersfrom the emitter to the base. The collector is reversed biased via thecontact pads with a voltage potential (base-collector voltage) thatcauses charge carriers to be attracted from the base to the collector.The corresponding currents are called the emitter current, base currentand the collector current.

FIG. 2 shows a general configuration of a bipolar transistor as avertical stack geometry. Typically the structure can be grownepitaxially, ion implanted or fabricated by various means. For avertical bipolar transistor, typically a conducting or insulatingsubstrate 0201 is used as the seed crystal to start the growth of thestructure. A highly conducting sub-collector 0202 is then grown,followed by a low doped collector 0203. The base 0204 which is ofopposite conductivity as the collector 0203 is then grown, followed byan emitter 0205, and finally a highly conducting contact layer 0206.Electrical contact is made to device via the metalized contact pads:emitter contact 0207, base contact 0208, and collector contact 0209. Thevoltages and currents are applied to the device via the contact pads.Vertical configuration offers some advantages.

Some of the advantages of a bipolar device are: typically in an NPNconfiguration electrons will travel vertically in the device from theemitter to the collector. Thus it is straightforward to produce deviceswhere the electron transit time through the device is short (high cutoff frequency F_(t)). Generally the entire area of the emitter contactwill conduct the current; thus one can have high current densities in asmall area, thus allowing for high circuit densities. The turn-onvoltage (voltage across the base-emitter junction) V_(BE) is independentof device processing issues like size because it corresponds to thepotential at the base-emitter junction, thus process variations across awafer can be minimized which is desirable for manufacturing. The turn-onvoltage V_(BE) controls the output current at the collector I_(C) andresults in a high transconductance g_(m)=eI_(C)/k_(B)T, where “e” is thecharge of the electron, I_(C) is the collector current, “k_(B)” isBoltzmann's constant, and “T” is the temperature. This is the highesttransconductance available for any three terminal device and allowscircuit operation with low V_(BE) swings.

The operation of the bipolar transistor (transistor action) is based onthe flow of charge carriers injected from the emitter into the basewhich can diffuse into the collector forming the emitter to collectorcurrent (collector current). The free charge carriers initially in theemitter are called majority carriers. The majority charge carriers thatare injected into the base from the emitter, once in the base, arecalled minority carriers, which then can diffuse to the collector. Thebase region controls the flow of the minority carriers injected into thebase thus controlling the flow of the collector current from the emitterto the collector. By drawing out the minority carriers that are injectedinto the base from the emitter, small levels of minority carriers drawnfrom the base control the larger collector current that flows from theemitter to the collector. Also, the base region is made thin to enhancethe diffusion of carriers from the emitter to the collector.

The current gain or “beta” of the bipolar transistor is the ratio of thecollector current I_(C) to the base current I_(B). Basically, the ratiois the number of carriers that get across the transistor from theemitter to the collector, vs. the number of carriers that get caught inthe base.

For a typical NPN transistor, the biasing scheme is as such. The emitterto base V_(BE) bias is such that the base is biased slightly positive ascompared to the emitter. The collector to base V_(BC) bias is such thatthe collector is biased much more positively than the base. This biasingscheme can ensure that, for small values of base current, large valuesof collector currents can be controlled. The current gain is typicallyabout “100”.

For a typical PNP transistor the biasing scheme is as such. The emitterto base V_(BE) bias is such that the base is biased slightly negative ascompared to the emitter. The collector to base V_(BC) bias is such thatthe collector is biased much more negatively than the base. This biasingscheme can ensure that, for small values of base current, large valuesof collector currents can be controlled.

The bipolar homojunction transistor can be made of one semiconductormaterial. The bipolar transistor can be used as a switch, amplifier, oroscillator, etc. It can be fabricated in discrete (single) component oras a component in integrated circuits.

Heterojunction bipolar junction transistors (HBT) can differ from thebipolar transistor (also called a homojunction bipolar transistor) byusing at least two different semiconductors. The heterojunction bipolartransistor typically uses different semiconductor materials for at leastone of the junctions, the emitter-base junction, and/or thebase-collector junction. The use of differing semiconductor materials iscalled a heterojunction.

Heterojunction bipolar transistors (HBTs) can be advantageous in somesituations for formation of the emitter-base junction. In homojunctions,transistors the emitter is typically doped (impurity incorporation withan atomic element to create free charge carriers) more heavily than thebase region. If the heterojunction is designed properly, the emitter hasan energy band gap greater than the base region. If the conduction andvalence band alignment of the two materials that form the heterojunctionis proper, it is then possible to limit the injection of majoritycarriers (initial free charge carriers in the base) of the base regioninto the emitter region (or additionally termed, limit the minoritycarrier injection into the emitter). This occurs because theheterojunction can create a potential barrier either in the valence orconduction band to block the majority carriers in the base, thuseliminating injection of majority carriers from the base into theemitter. In the heterojunction transistor, the base can be heavily dopedat concentrations much greater than the emitter material. The physics ofthe heterojunction can be strongly determined by the conduction andvalence band alignment between the materials.

FIG. 3 shows a flatband energy diagram for three typical heterojunctionsituations between emitter and base materials (focuses on the case wherewe have an N-type emitter material and a P-type base material): type I,type II, near zero conduction band offset. The heterojunction is at theinterface between the emitter material and the base material. The typeof diagram depicted is called a flat band energy diagram, whichrepresents how the conduction band edge 0310 and the valence band edge0311 change as one goes through the dis-similar semiconductor materials.The vertical axis 0312 has the value of Energy with typical units of“eV”, and the horizontal axis 0313 is the relative Distance in arbitraryunits “A.U.” through the heterojunction of semiconductor materials. Theenergy level line called the conduction band edge 0310 is the minimumenergy value of the conduction band, and the energy level line calledthe valence band edge 0311 is the maximum value of the valence band. Thediagram shows distance on the horizontal scale and that is a relativedistance into the semiconductor device. One could put units ofthickness, but that is usually omitted, and this represents a schematicfor carrier transport. The difference between the conduction band edge0310 and the valence band edge 0311 in the base material is the band gapenergy of the base material. The difference between the conduction bandedge 0310 and the valence band edge 0311 in the emitter material is theband gap energy of the emitter material.

There are various types of heterojunctions between the emitter and basematerials: (1) Type I heterojunction 0301; (2) Type II heterojunction0302; and (3) near zero conduction band offset 0303 (there can also be anear zero valence band offset) at the heterojunction. Type Iheterojunction has an energy discontinuity at the conduction band andvalence band, where the smaller band gap at base material 0305 liesbetween the conduction and valence band edges of emitter material 0304.ΔE_(C) is called the conduction band offset at the emitter-baseheterojunction (difference between the conduction band edges in therespective materials), and ΔE_(V) is call the valence band offset at theemitter-base junction (difference between the valence band edges in therespective materials). Type II heterojunctions have a discontinuity atthe conduction and valence band edge, but the base energy alignment isstaggered or offset. The energy band gap of base material 0307 can bestaggered above emitter material 0306, and the band gap as depicted inthe figure (or staggered below the emitter material 0306 band gap).Finally, one can have a situation of a zero or near zero conduction bandoffset heterojunction 0303 as shown in the figure, where the conductionband offset ΔE_(C) is zero or small, typically less than 0.1 eV betweenemitter material 0308 and base material 0309.

For NPN heterojunction transistors, where the emitter material is N-typeand the base material is P-type, a large valence band offset ΔE_(V)between the emitter and the base is desired, as shown in the three cases(1) Type I heterojunction 0301; (2) Type II heterojunction 0302; and (3)near zero conduction band offset heterojunction 0303. This large valenceband offset ΔE_(V) prevents the back injection of holes from the base toemitter, which can reduce the gain of the transistor. Thus the basematerial band gap energy should be less than the emitter material bandgap energy. Looking at the FIG. 3 diagram, it seems that a type IIheterojunction current 0302 would be preferable because it has thelargest valence band offset ΔE_(V), but in some examples, the desiredsituation for the efficient transport of carriers across the base topromote transistor action is the near zero conduction band offsetheterojunction 0303 situation or where the conduction band offset ΔE_(C)is typically less than 0.1 eV.

FIG. 4 shows a flatband energy diagram for three typical heterojunctionsituations between the base material and the collector material (focuseson the case where we have a P-type base material and an N-type collectormaterial): type I, type II, near zero conduction band offset. Theheterojunction is at the interface between the base material andcollector material. The type of diagram depicted is called a flat bandedge energy diagram, where the vertical axis is the Energy (eV) 0412value and the horizontal axis is a relative Distance (A.U.) 0413 throughheterojunction of semiconductor materials. The energy level lines arecalled the conduction band edge 0410, is the minimum energy value of theconduction band and the valence band edge 0411 is the maximum value ofthe valence band. The diagram shows distance on the horizontal scale,and the distance is a relative distance into the semiconductor device.One could put units of thickness, but that is usually omitted and thisrepresents a schematic for carrier transport. The difference between theconduction band edge 0410 and the valence band edge 0411 in the basematerial is the band gap energy of the base material. The differencebetween the conduction band edge 0410 and the valence band edge 0411 inthe collector material is the band gap energy of the collector material.

There are various types of heterojunctions between the base andcollector materials: (1) Type I heterojunction 0401; (2) Type IIheterojunction 0402; and (3) near zero conduction band offset 0403(there can also be a near zero valence band offset) at theheterojunction. Type I heterojunction has an energy discontinuity at theconduction band and valence band, where the smaller band gap basematerial 0404 regions lies between the conduction and valence band edgesof the collector material 0405. ΔE_(C) is called the conduction bandoffset at the base-collector heterojunction (difference between theconduction band edges in the respective materials), and ΔE_(V) is callthe valence band offset at the base-collector heterojunction (differencebetween the valence band edges in the respective materials). Type IIheterojunctions have a discontinuity at the conduction and valence bandedge, but the base energy alignment is staggered or offset. The energyband gap of base material 0406 can be staggered above the band gap ofcollector material 0407 as depicted in the figure (or staggered belowthe band gap of collector material 0407). Finally, one can have asituation of a zero or near zero conduction band offset heterojunction0403 as shown in the figure, where the conduction band offset ΔE_(C) iszero or small, typically less than 0.1 eV between the base material 0408and the collector material 0409.

For NPN heterojunction transistors, where the base material is P-typeand the collector material is typically N-type, one would like a largecollector band gap energy because this allows the NPN transistor to havea big breakdown voltage. Looking at the FIG. 4 diagram, it seems that atype II heterojunction 0402 would be preferable because the basematerial 0406 has higher conduction band energy than the collectormaterial 0407 but, in some examples, the desired situation for theefficient transport of carriers across the base to promote transistoraction is the near zero conduction band offset heterojunction 0403situation or where the conduction band offset ΔE_(C) is typically lessthan 0.1 eV.

FIG. 5 shows the flatband energy diagram through an NPN heterojunctionbipolar transistor 0500 for optimized transport. The figure shows thelineup of the conduction band edge 0504 and the valence band edge 0505through the NPN heterojunction transistor 0500. There is a zeroconduction or near zero conduction band edge offset ΔE_(C) from the Nemitter material 0501 to the P base material 0502 to the N collector0503. The emitter-base valence band offset is represented by ΔE_(VE) andthe base-collector valence band offset is ΔE_(VC). In physicalsituations it is desirable to have the smallest conduction band offsetthat is possible from N emitter material 0501 to P-base material 0502 toN collector 0503. Here the band gap energy of N emitter material 0501 islarger than the band gap energy of P base material 0502, where there isa large valence band offset ΔE_(VE) between N emitter material 0501 to Pbase material 0502, and the junction is a heterojunction. The band gapenergy of N collector material 0503 can be less than, equal to, orgreater than the band gap energy of P base material 0502. Generally theband gap energy of N collector material 0503 should be equal to, orgreater than, the band gap energy of P base material 0502. The greaterthe band gap energy of N collector material 0503 the better thebreakdown voltage of NPN heterojunction bipolar transistor 0500. This isgenerally desirable for high power and robust devices. If N collectormaterial 0503 is the same material as P base material 0502, there is ahomojunction at the base-collector junction and a heterojunction at theemitter-base junction, and such a device is called a singleheterojunction bipolar transistor device. If N emitter material 0501 andN collector material 0503 are the same, then the device is called asymmetric double heterojunction bipolar transistor device, and such adevice results in a minimum in zero offset voltage in the measurement ofthe collector current vs. the collector-emitter voltage as a function ofthe stepped voltage bias of the base-emitter junction, which isdesirable to improve the power added efficiency of NPN heterojunctionbipolar transistor 0500. For robust and high power, one would like thecollector band gap energy to be as large as possible. If the emittermaterial, the base material, and the collector material are alldis-similar, the device would be called an asymmetric doubleheterojunction bipolar transistor device.

The NPN heterojunction transistor can promote efficient transport whenthere is zero or near zero or the smallest conduction band offsetbetween emitter-base-collector. In an NPN device, the electrons are keycarrier that makes up the collector current, and the base-emitterjunction controls this electron current. The base alignment of theconduction band offset can be desirable in some examples. Largeconduction band energy offsets or discontinuities at the emitter-base orcollector-base junctions can hinder electron transport.

FIG. 6 shows the flatband energy diagram through a PNP heterojunctionbipolar transistor 0600 for optimized transport. The figure shows theline-up of conduction band edge 0604 and valence band edge 0605 throughPNP heterojunction transistor 0600. There is a zero valence band or nearzero valence band edge offset ΔE_(V) from P emitter material 0601 to Nbase material 0602 to P collector 0603. The emitter-base conduction bandoffset is represented by ΔE_(CE) and the base-collector conduction bandoffset is ΔE_(CC). In physical situations it is desirable to have thesmallest valence band offset that is possible from P emitter material0601 to N base material 0602 to P collector 0603. Here the band gapenergy of P emitter material 0601 is larger than the band gap energy ofN base material 0602, where there is a large conduction band offsetΔE_(CE) between P emitter material 0601 to N base material 0602, and thejunction is a heterojunction. The band gap energy P collector material0603 can be less than, equal to, or greater than the band gap energy ofN base material 0602. Generally the band gap energy of P collectormaterial 0603 should be equal to, or greater than, the band gap energyof N-base material 0602. The greater the band gap energy of P collectormaterial 0603 the better the breakdown voltage of PNP heterojunctionbipolar transistor 0600. This is generally desirable for high power androbust devices. If P collector material 0603 is the same material as Nbase material 0602, there is a homojunction at the base-collectorjunction and a heterojunction at the emitter-base junction, and such adevice is called a single heterojunction bipolar transistor device. If Pemitter material 0601 and P collector material 0603 are the same, thenthe device is called a symmetric double heterojunction bipolartransistor device, and this results in a minimum in zero offset voltagein the measurement of the collector current vs. the collector-emittervoltage as a function of the stepped voltage bias of the base-emitterjunction, which is desirable to improve the power added efficiency ofPNP heterojunction bipolar transistor 0600. For robust and high power,one would like the collector band gap energy to be as large as possible.If the emitter material, the base material, and the collector materialare all dissimilar the device would be called an asymmetric doubleheterojunction bipolar transistor device.

The PNP heterojunction transistor can promote efficient transport whenthere is zero or near zero valence band offset betweenemitter-base-collector. In a PNP device, the holes are key carriers thatmake up the collector current, and the base controls this hole current.The base alignment of the valence band offset can be desirable in someexamples. Valence band energy discontinuities at the emitter-base orcollector-base junctions can hinder hole transport.

The relationship of the conduction and valence band offsets for manysemiconductors is well studied, and numerous values of the conductionband offsets ΔE_(C) and valence band offsets ΔE_(V) between dissimilarsemiconductors (heterojunction) have been published in the literature.

Unlike homojunction bipolar transistors, heterojunction bipolartransistors (HBTs) allow for a higher base doping density (>1×10¹⁹ cm⁻³)thus reducing the base resistance and maintaining current gain. For NPNHBTs, higher base doping density can occur as a result of the largevalence band offset at the emitter-base junction. For PNP HBTs, higherbase doping density can occur as a result of the large conduction bandoffset at the emitter-base junction.

Typically, one would like to have the highest doping density that ispossible in the base. Typically the highest levels of base doping aregreater than 1×10¹⁹ cm⁻³. High doping levels are typically greater than1×10¹⁸ cm⁻³ range and typically low doping levels are in the 1×10¹⁶ cm⁻³to 5×10¹⁷ cm⁻³ range. The high doping density in the base causes areduction in the base sheet resistance thus allowing the transistor tohave larger F_(max) (e.g., the maximum frequency to get power gain outof the transistor). Also, by having high base doping one can reduce thethickness of the base and increase the F_(t) (e.g., the transitfrequency, time for carrier to go across base region). The relationshipbetween transit frequency F_(t) and the maximum oscillation frequencyF_(max) is as follows for an HBT: F_(max)=(F_(t)/8πR_(B)C_(CB))^(1/2).The transit frequency F_(t) is basically inverse of the time for theelectron to traverse the emitter, base and collector. The parametersR_(B) and C_(CB) refer to the base sheet resistance and the capacitanceof the collector-base junction. The parameter F_(max) is the unity powergain frequency and indicates the maximum frequency with power gain froma device.

The reason why heterojunction bipolar transistors (HBTs) can beadvantageous, is that heterojunction bipolar transistors (HBTs) allowfor a higher base doping density (>1×10¹⁹ cm⁻³) thus reducing the baseresistance and maintaining current gain. For various choices of theemitter and base materials, it is possible to obtain large valence bandoffset ΔE_(V). This large valence band offset ΔE_(V) prevents the backinjection of minority carriers into the emitter, thus keeping the gainof the HBT high (no degradation of the gain with high doping of the basematerial).

Additionally, in some examples for the base-collector junction, thebase-collector breakdown voltage is set by the energy band gap of thecollector material. Typically, one would like to have a low energy bandgap base material (typically these are relevant semiconductors with bandgaps less than 0.75 eV, like GeSn, Ge, InGaAs, GaAsSb) because that setsthe turn-on voltage of the base-emitter junction—or the onset oftransistor action. However, in a homojunction (base and collectormaterials are the same), a low energy band gap at the collector canresult in a low base-collector breakdown voltage. Thus a large potentialdifference between the base and the collector junction could allow thetransistor to have a low breakdown voltage which causes the transistorto be easily damaged thus hurting ruggedness. In a heterojunctionbipolar transistor, it is possible to combine a low energy band gap baseregion with a large energy band gap collector region thus allowing for alarge breakdown voltage. Heterojunctions transistor can be optimizedutilizing optimized material for the emitter, base, and collector.Heterojunction bipolar transistors can optimize the emitter basecollector regions to make high performance and high power transistors.

The characteristics and properties of the base and the base-emitterjunction and the base-collector junction are the dominant factors thatdetermine the electronic properties of bipolar and heterojunctionbipolar transistors. Thus the utilization of a new type of base materiallike GeSn in these transistors allows for the development of vastlyimproved transistors for high speed and power efficient operation.

GeSn is a useful material for use as the base material for bipolartransistors because it can become a direct gap material at higher Sn %,which makes it a useful material for light emission. Thus GeSn in bulkform or GeSn quantum wells (QW) or GeSn quantum wires, or GeSn quantumdots (QD) can act as the active region for light emission in devicessuch as a light emitter, laser or transistor laser.

Lasers are devices that can produce intense narrowly divergent,substantially single wavelength (monochromatic), coherent light. Laserlight of different wavelengths can be advantageously applied in manyfields, including biological, medical, military, space, industrial,commercial, computer, and telecommunications.

Semiconductor lasers may utilize an active region, which may be formedwith a homojunction (using similar materials), single or doubleheterojunction (using dissimilar materials), or with a quantum dot(“QD”), quantum wire, quantum well (“QW”) or quantum cascade region. Theenergy transitions can occur from interband or inter-sub-band electronicstates. The quantum well or quantum wire, or quantum dot structure maybe formed when a low energy band gap semiconductor material is typicallysurrounded or confined by a larger bandgap semiconductor materials.Additionally, these quantum confined heterostructures can be type I, ortype II or type III (broken energy alignment). The fundamentalwavelength that characterizes quantum well (QW) or quantum dot (QD) isdetermined primarily by the thickness, composition, and material of thequantum well.

In order for lasing to occur, a laser device typically has a resonantcavity and a gain medium to create population inversion. In some highlyefficient semiconductor laser examples, population inversion generallyoccurs with the injection of electrical carriers into the active region,and the resonant cavity is typically formed by a pair of mirrors thatsurround the gain medium. The method of injection of carriers can bedivided into electrical injection of carriers and optical pumping forinjection of carriers. Electrical injection is generally performed by anelectrical current or voltage biasing of the laser and forms the basisof the electrical injection laser. Optical pumping typically usesincident radiation that allows the formation of electrons and holes inthe laser. Additionally, these methods can be operated in a continuouswave (CW) pulsed, synchronous, or asynchronous modes.

FIG. 7 shows the general configuration of a PN junction laser orinjection laser diode 0700, with a quantum well or quantum dot 0703active region. This PN junction device operates on the principle ofminority injection of carriers (electrons and holes, the I_(diode)current) into the active region and waveguide 0705. The P⁺ cladding 0701region may serve as the injection of holes. N⁺ cladding 0706 may serveas the injection of electrons. It can be possible when a low band gapmaterial is placed inside a larger energy band gap, like the opticalconfinement layers OCL 0702 and OCL 0704, the formation of QW or QD 0703can be formed. These QW or QD 0703 can serve as the active region forthe collection of both electrons and holes and produces the invertedpopulation necessary for laser operation. The wide band gap P⁺ cladding0701 and N⁺ cladding 0706 semiconductors provide for the opticalconfinement because their index of refraction is generally lower thanthat of the optical confinement materials OCL 0702 and OCL 0704. Thecladding layers also provide funneling of the electrical carriers to theQW or QD 0705 regions. Light 0707 can be produced by recombination ofcarriers in the QW or QD. Additionally, there are the opticalconfinement layers (OCL) which serve as the barrier to the QW or QDregion thus providing for the quantum confinement, and also serves asthe waveguide material. The OCL layers generally have band gap energiesbetween that of the QW or QD and the wide band gap energy claddinglayers. The combination of the QW or QD 0703 and the OCL 0702 and OCL0704 form the active region and waveguide 0705 of the laser structure.

FIG. 8 shows the exemplary flat band energy diagram showing theconduction band edge 0808 and the valence band edge 0809 of a separateconfinement heterostructure (SCH) laser 0800 with an active QW or QD0803 region. The OCL1 0802 and OCL2 0804 form the barrier layers to theQW or QD 0803 which allows for quantum confinement and can be of thetype I heterojunction band alignment. Such a structure providesefficient recombination of the carriers and good optical confinement ofthe light produced from the recombination of the carriers. The P⁺cladding large bandgap 0801 and the N⁺ cladding large bandgap 0806provides for minority carrier injection and funnels the carriers intothe waveguide region ultimately recombining in the QW or QD 0803 activeregion. The cladding layers form the boundary for the waveguide 0807with the OCL1 0802 and OCL2 0804 regions thus providing for efficientconfinement of light. In this design the P⁺ cladding large bandgap 0801and N⁺ cladding large bandgap 0806 have the largest bandgap energies inthe device structure. Next the OCL1 and OCL2 layers have the nextlargest bandgap energies. Finally the QW or QD 0803 materials have thesmallest bandgap energies. In this design the P⁺ cladding large bandgap0801 and OCL1 0802 has a type I heterojunction alignment, and in thisdesign the N⁺ cladding large bandgap 0806 and OCL2 0804 also has a typeI heterojunction alignment. Various configuration of the SCH arepossible for optimization of the laser. Note that though the figureshows only one QW or QD region, but multiple QW or QD regions can beused, if higher efficiencies are wanted. Typically such a structure isinserted into a resonant cavity for the light amplification that isrequired for laser operation.

Two common types of semiconductor lasers are in-plane, also known asedge emitting or Fabry Perot lasers (also includes distributed feedbacklasers), and surface emitting also known as vertical cavity surfaceemitting lasers (“VCSELs”). Edge emitting lasers emit light from theedge of the semiconductor wafer whereas VCSELs emits light from thesurface of the laser. In addition, for the edge emitter the resonantcavity is typically formed with cleaved mirrors at each end of theactive region.

FIG. 9 is a perspective schematic of a typical edge emitting injectiondiode laser or in-plane semiconductor laser 0901. The edge emittinglaser 0901 can comprise a substrate 0910 with an active region 0906disposed between a P-type cladding layer 0904 and an N-type claddinglayer 0905. Cleaved facets on the front 0908 and on the back 0909 of thelaser typically form a resonant optical cavity. The order of the layersmay not be restricted as described above. To activate the laser, a biascurrent can be applied to top 0902 and bottom 0903 metal contacts. Uponapplication of the bias to the laser, light of a wavelength λ 0907 istypically emitted from the edge of the laser. An exemplary edge emittingsemiconductor laser may include a GeSn quantum well active regionmaterial 0906 with GaAs barriers for near-infrared (IR) light emission0907, or their equivalents. The QW active region of the structure istypically capable of emitting a designed center wavelength over a widerange of possible wavelengths depending on a number of device designparameters including but not limited to the thickness and composition ofthe layer materials. Being able to tune light over the wide range ofwavelengths could be useful for a variety of applications.

The design and fabrication of this type of edge emitting laser structuremay utilize consideration of the material properties of each layerwithin the structure, including energy band structure and bandalignments, electronic transport properties, optical properties, systemsdesign, and the like. An edge emitting laser such as the exemplary onedescribed above may satisfactorily be wavelength tuned in the mannerpreviously described.

It may also be possible to omit the top metal 0902 and the bottom metal0903 and optically pump the edge emitter from the top, bottom or sidewith another laser that may have an emission wavelength shorter than theedge emitter 0901. This may simplify the process because metallizationof the laser 0901 can be avoided.

The second type of semiconductor laser, VCSELs, emits light normal tothe surface of the semiconductor wafer. The resonant optical cavity of aVCSEL may be formed with two sets of distributed Bragg reflector (DBR)mirrors located at the top and bottom of the laser, with the activeregion (which may be a quantum well or quantum wire, quantum dotregion), sandwiched between the two Bragg reflectors. Note thedesignation of N DBR means that the DBR is doped N-type.

FIG. 10 is an exemplary schematic of the side view of a vertical cavitysurface emitting laser (“VCSEL”) 1001. A substrate 1006 may havedeposited layers of P-type distributed Bragg reflectors (“DBR”) material1003, and N-type distributed Bragg reflector material 1005. An activeregion 1004 is inserted in the optical confinement layer 1009 thensandwiched between the P DBR 1003 and the N DBR 1005 structures. Metalcontacts 1002, 1007 are provided for applying a bias to the laser. TheP-type DBRs 1003 and N-type DBRs 1005 form the resonant optical cavity.The order of the layers is not restricted as described above. Uponapplication of a current bias to the laser, light 1008 is typicallyemitted from the surface of the laser, which can be the bottom or top ofthe laser. A VCSEL laser 1001 such as the exemplary one described abovemay allow laser emission from the surface of the structure rather thanfrom the side, or edge, as in the edge emitting laser of FIG. 9. Thoughthe diagram of an example of a VCSEL shows the light is coming out ofthe bottom, but could be designed so that light comes out of the top.

It may also be possible to omit the top metal 1002 and the bottom metal1007 and optically pump the VCSEL 1001 from the top or bottom withanother laser that may have an emission wavelength shorter than theVCSEL 1001. This may simplify the process because metallization of thelaser 1001 can be avoided.

These lasers can be called diode lasers or injection diode lasers andare two terminal devices. The HBT are three terminal devices. It ispossible to combine both structures to form the light emittingheterojunction bipolar transistor which can act as a three terminaldevice, but also can emit light. Such a device would allow forintegrated circuit designs that could transmit data optically and act ashigh speed switching transistors, all in a single device. Because thelight emitting transistor is a three terminal device, the extra terminalallows the biasing of the base collector junction to quickly collect thecharge carriers and, thereby out performing laser diodes and/or twoterminal devices.

For both the edge emitting laser 0901 and the VCSEL 1001, the inputcontrol to the lasers may be a current bias, voltage bias or opticalpump techniques as described. Furthermore, both electrical injection andoptical pumping can be operated in continuous wave (CW), pulsed,synchronous, or asynchronous modes of operation.

The light emitting transistor laser could comprise a bipolar transistorwith a direct gap quantum well or quantum dot or quantum wire insertedin the base/barrier region. The quantum well or quantum wire or quantumdot forms the collection region (active region) for electrons and holesto recombine to generate light.

In the following figures or tables, N⁺ refers to high N-type doping, N⁻refers to moderate N-type doping, P⁺ refers to high P-type doping, P⁻refers to moderate N-type doping, and UID refers to unintentionaldoping.

FIG. 11 shows a simple diagram of an NPN transistor laser 1100 showingthe three terminal device configuration with the correspondingemitter-base-collector designation, with a quantum well or quantum dotactive 1103 inserted into P base/barrier 1102 and 1104. Here emitter1101 and the collector 1105 can form the cladding regions for opticalconfinement of the light 1107 produced by recombination of the carriersin the quantum active region. The P base forms the barrier region forthe quantum well, quantum wire or quantum dot and also the waveguidematerial. Because the transistor laser acts as a transistor and a laser,the emitter-base-collector needs to have dual functions for theelectrical and optical. For an NPN HBT laser the emitter has to injectelectrons into the base and also form the cladding layer for lightconfinement as designated by N emitter/cladding 1101. The base regionshould be heavily P-type doped and forms the barrier to the QW or QD1103 material and thus is designated by the P base/barrier 1102 and Pbase/barrier 1104. The P base/barrier 1102 and 1104 and the QW or QD1103 form the active region and the waveguide 1106 of the NPN transistorlaser 1100. To additionally form for example a light edge emittingtransistor laser a resonant cavity is typically formed by cleavingmirrors at the front and back facets of the crystal to optically amplifythe photon population

FIG. 12 shows a cross-sectional device depiction of an NPN edge emittingtransistor laser 1200. The device comprises an N⁺ substrate 1201 as thestarting point. A heavily doped N⁺ sub-collector 1202 is grown on the N⁺substrate 1201. Then a dual purpose lightly doped N− collector/bottomcladding 1203 layer is formed. Then a P⁺ base/barrier/opticalconfinement layer 1204 is grown and this also starts the formation ofthe waveguide region for the confinement of the light. Next the QW or QD1205 active region is grown, and then on top of this layer is the P⁺base/barrier/optical confinement layer 1206 which finishes the waveguideportion of the device. On top of this layer is then grown the N⁻emitter/upper cladding 1207 which provides the cladding finishing up thedevice. Metal contacts are placed at the N⁺ emitter contact 1208 via thetop metal 1209 contact, the P⁺ base/barrier/optical confinement layer1206 via the base metal 1210, and the N⁺ substrate 1201 via the bottommetal 1211. The resonant cavity is formed by the front cleaved crystalfacets 1214 and the back cleaved crystal facets 1213. Light 1212 isemitted from the front of the device. The device is biased in a standardNPN transistor configuration, but because of the inclusion of the QW orQD 1205, P⁺ base/barrier/optical confinement layer 1204 and 1206 and theN-collector/bottom cladding 1203 and N⁻ emitter/upper cladding 1207, thedevice will form a light emitting or laser transistor when properlydesigned. A typical way of making this structure is to use epitaxialtechniques to grow the basic layered structure then use standard deviceprocessing techniques to fabricate the full device. The laser includesquantum well or quantum wire or quantum dot inserted into a base regionof the heterojunction bipolar transistor. The laser can require aresonant cavity to get optical gain, and typically this can formed fromthe front and back cleaved facets of the semiconductor crystal wafer.

Vertical emission of light normal to the surface of the semiconductorwafer is also a useful configuration for transistor lasers. The resonantoptical cavity of a vertical transistor laser may be formed with twosets of distributed Bragg reflector (DBR) mirrors located at the top andbottom of the laser, with the active region (which may be a quantum wellor quantum wire, quantum dot region), sandwiched between the two Braggreflectors.

FIG. 13 shows a cross-sectional device depiction of an example of apossible configuration of a NPN VCSEL transistor 1300, where the lightis coming out of the top, but also could be designed so that light comesout of the bottom. The device can be grown on an N⁺ conducting substrate1301, with the growth of a bottom N⁺ DBR 1302 stack which forms thebottom mirror of the device. N⁻ collector 1303 is then grown on thebottom mirror. P⁺ base 1304 is then grown on the N⁻ collector 1303, andalso forms the barrier for QW or QD 1305 active region. QW or QD 1305 isdeposited on P⁺ base 1304, and then P⁺ base 1306 is deposited on QW orQD 1305, finalizing the barrier material to the active region forquantum confinement effects. N⁻ emitter 1307 is then grown on P⁺ base1306. Then an N⁺ contact 1308 is deposited on N⁻ emitter 1307. Finally adielectric mirror stack D DBR 1309 is deposited on N⁺ contact 1308.VCSEL transistor laser 1300 is processed using standard techniques ofmesa etch and metallization to form the final structure, where metalcontacts emitter metal/aperture 1311 is deposited on N⁺ contact 1308.Emitter metal 1311 forms an aperture for light out 1310. Base metal 1312is deposited on P⁺ base 1306, and bottom metal 1313 is deposited on N⁺substrate 1301. NPN VCSEL transistor laser 1300 is operated as astandard bipolar device.

GeSn is alloy semiconductor of the constituent semiconductors germanium(Ge, which is an indirect semiconductor, with an energy band gap of 0.66eV) and alpha tin or cubic tin (Sn which is zero energy gap directsemiconductor). GeSn can be an indirect or direct energy band gapsemiconductor depending on the alloy composition. A direct gapsemiconductor has its conduction band minimum energy and valence bandenergy maximum occur at the same crystal momentum (k-space). If thelocation of the conduction band energy minimum and the valence bandenergy maximum occurs at different crystal momentum (or differentlocation in k-space), it is an indirect semiconductor. Direct gapsemiconductors are highly efficient for radiative recombination ofelectrons and holes, thus most light emitting devices are fabricatedfrom direct gap semiconductors.

GeSn semiconductors have been grown epitaxially by metalorganic chemicalvapor deposition (MOCVD), molecular beam epitaxy (MBE), ion implantationfollow by anneal, by pulse laser deposition (laser ablation), but,additionally, liquid phase epitaxy, vapor phase epitaxy and variousother epitaxial growth techniques can be used to grow the GeSn materialdescribed herein. The GeSn layers have been grown up to 20% Sn content.Both N-type and P-type doping has been achieved in GeSn layers.

FIG. 14 shows the energy band structure diagrams for the semiconductorsGe 1401 and Sn 1402. The vertical axis is the Energy with units of (eV)and the horizontal axis is the Wavevector k. The energy band structurediagrams depict the available or unavailable (forbidden gap) energylevels for the charge carriers in the semiconductor. The plot showsenergy on the vertical scale and the crystal momentum direction on thehorizontal scale. There are significant crystal momentum points “X,” “Γ”(Brilloun zone center) and “L.” Ge 1401 is an indirect gap semiconductorbecause the conduction band minima is at the “L” point and the valenceband maximum is at the “r” point. Sn 1402 is a semimetal or zero directgap semiconductors with the conduction band minima and valence bandmaxima at the “Γ” point. As Sn is added to Ge, the conduction bandenergy at the “Γ” point moves down faster than the conduction bandenergy at the “L” point, and thus, turning GeSn at some alloycomposition turns into a direct gap semiconductor.

The direct to indirect transition can occur about 7% Sn in GeSn, but canbe observed up to 11% Sn. The energy band gap at 7% Sn is about 0.585eV, this corresponds to a wavelength of about 2370 nm. Ge is a group IVsemiconductor and though it is an indirect semiconductor, it has someproperties that are advantageous. Ge 1401 has a local minimum at the “Γ”point of the conduction band. The lowest energy point in the Ge 1401conduction band is at the “L” point and is only 0.14 eV lower than the“Γ” point a room temperature. Various methods can be used to lower thegamma point below the L point such as introducing biaxial tensilestrain, or heavily N-type doping the Ge. However, by adding Sn to Ge, itis possible to lower the band gap but also form a direct gapsemiconductor. In addition, one could employ both tensile strain andadding Sn to Ge to make a direct gap semiconductor.

In some examples heterojunction bipolar transistors (HBT) can be adesirable device for greater power handling capability, higher powerefficiency, and lower signal distortion in some examples. Thefabrication of a GeSn based HBT structure enables a new transistortechnology that can significantly outperform SiGe, GaAs, InP, and GaNtransistors in high-power, high-frequency applications. The new HBTsemiconductor structure described herein exhibits a large valence banddiscontinuity between the emitter and base; has a low energy band-gapbase (the term low energy band gap base typically refers to the relevantsemiconductors with band gaps less than 0.75 eV, like GeSn, Ge, InGaAs,GaAsSb); and a second (double) heterojunction can be inserted betweenthe base and collector with a good breakdown electric field. Theseattributes positively can impact several key device parameters such ascollector-emitter breakdown voltage, DC current gain, and power gaincutoff frequency (F_(max)). The low band-gap GeSn base can significantlydecrease transistor turn-on voltage and thereby increase the power addedefficiency of the device.

FIG. 15 shows a graph of the collector current density J_(C) vs. theturn-on voltages (V_(BE)) of various HBT material systems. The figureshows a plot of the collector current density J_(C) (A/cm²) verticalscale versus base-emitter voltage V_(BE) (V), horizontal scale. Theplotted characteristics (ideal) for several different heterojunctionbipolar transistor (HBT) technologies are shown. The GeSn HBT structuredescribed herein has the lowest turn-on voltage 1501 of the technologiesshown of Ge, InP/InGaAs, SiGe, GaAs, GaN/InGaN.

For NPN heterojunction transistors, it is generally desirable for thebase region to be heavily P-type doped. This allows for the base sheetresistance to be minimized thus allowing for high frequency operation ofthe transistor. The GeSn base can be heavily doped P-type in someembodiments. When the hole concentration as measured by Hall Effect,high doping levels (>1×10¹⁹ cm⁻³) can be achieved in GeSn.

FIG. 16 shows the hole concentration of GeSn films as a function of Sn%, as measured by Hall effect. The vertical axis is the holeconcentration (cm⁻³) and the horizontal axis is the Sn %. From the holeconcentration data vs. Sn % of FIG. 16, it is readily seen that GeSn canbe P-type doped at the highest levels of base doping which are greaterthan 1×10¹⁹ cm⁻³. Additionally, GeSn can achieve a large hole mobility(Ge hole mobility is about 1800-2000 cm²/Vs, as compared to GaAs holemobility 400 cm²/Vs) which is a precondition for making the base regionthin.

By utilizing GeSn in the base of a heterojunction bipolar transistor,one can lower the turn-on voltage because the bandgap of GeSn is lessthan that of Ge, which is less than that of the materials systems GaN,GaAs, Si, InP, GaP, AlAs, and Ge. Additionally, at low Sn content, GeSnhas all the advantages that a Ge base material adds. For an NPNstructure, Ge is desirable for the base region because it can be heavilydoped P-type, it has the highest hole mobility (desirable for reducingthe resistance of the base), also this hole mobility can be increased byapplying tensile or compressive strain, and its conduction bandalignment is favorable with numerous semiconductors. At about 7% Sn andgreater, GeSn goes from an indirect semiconductor to a direct gapsemiconductor, though this indirect to direct transition can occur at Snpercentages up to 11%.

FIG. 17 shows a representative graph showing the GeSn direct band gapenergy vs. lattice constant. The vertical axis shows the values of theEnergy Bandgap (eV) and the horizontal axis shows the Lattice Constant(Å). The dots and the line through the dots represent GeSn direct gapenergy as a function of its lattice constant. Because the data is forthe direct band gap transition, the first data point is near 7% Sn andthe last is near 20% Sn. Ge has an indirect bandgap energy of 0.66 eV.GeSn is indirect up to about 7% Sn then becomes a direct gapsemiconductor with an energy gap of 0.585 eV and a lattice constant ofabout 5.725 Å, and the band gap energy typically reduces to about 0.25eV at 20% Sn with a lattice constant of about 5.835 Å. The emissionwavelengths that can occur in bulk direct gap GeSn range from about 2370nm at 7% Sn to 5540 nm at 20% Sn.

FIG. 18 shows a replot of FIG. 17 showing a representative graph showingthe GeSn emission wavelength vs. lattice constant. The vertical axisshows the values of the Wavelength (nm) and the horizontal axis showsthe Lattice Constant (A). The dots and the line through the dotsrepresent GeSn possible emission wavelength as a function of its latticeconstant. For on-chip communications 1000 nm to 3000 nm is acceptable.For telecommunications applications, the typical wavelengths used are1300 nm and 1550 nm. These wavelengths can be achieved by using quantumwell or quantum dot GeSn materials. These low dimensional structureslike two dimensional “2D” QW, one dimensional “1D” quantum wires, orzero dimensional “0D” quantum dot structures increase the light emissionenergy due to quantum confinement effects.

It should be noted that GeSn is an alloy semiconductor and that Snpercentages can be varied from 0%≦Sn %≦20%.

GeSn materials are useful for quantum confined structures. Quantumconfined structures such as quantum wells (QWs), quantum wires, andquantum dots (QDs) structures add a new degree of freedom in makinglight emitting materials. Because GeSn can be become a direct gapmaterial at 7% to 11% Sn content, and it would be useful to havepossible wavelengths in the 1000 to 3000 nm range because this coversthe telecommunications and fiber optics networks. One method of takingGeSn bulk material to get energies that cover this wide range is to usequantum well, quantum wire, or quantum dot technologies, because thelight emission is then dependent on quantum confinement or quantum sizeeffects.

QDs form artificial semiconductor atoms with electronic “shells” thatcan be engineered to control their light absorption properties. Besidestheir novel electronic properties, QDs also have interesting materialproperties; their 3-dimensional shape allows greater strain relief atthe QD surfaces than for planar growth. GeSn QDs can be grown on Siwithout creating dislocations. Absorption over broad wavelengths comesfrom an ensemble of QDs that have sizes that vary statistically. Alsobecause GeSn at low Sn content starts as indirect material and thenbecomes a direct gap material at higher contents, it is possible toproduce efficient light emission in QD structures with indirect gapsemiconductors. The limitations of the indirect nature of the bandgapGeSn (Sn %<7%) can be overcome by the formation of low-dimensionalstructures such as quantum dots because this method uses the spread ink-space caused by the quantum confinement to circumvent the indirectbandgap problem of the GeSn. Thus QDs are useful for producing lightemission in direct and indirect gap materials.

The use of GeSn as the active layer of the quantum dot laser hassignificant advantages. The greater than 4% mismatch between GeSn and Siallows for the self-assembly of Ge islands by the Stranski-Krastanovgrowth mode (strained layer epitaxy). This 3-dimensional growth mode isa method of making zero dimensional structures (i.e. QD). For QDs toeffectively provide light emission, the QD material is generally of alower energy band gap than the barrier material. The relatively low GeSnbandgap energy makes it a desirable starting point for absorption in thenear-IRl and mid-IR. It is possible by controlling the size of the Gequantum dots, to change the interband (electron-hole recombination) toallow for energy transitions in the near-IR to mid-IR. The GeSn QD withSi barriers can be of type II band alignment.

FIG. 19 shows the formation of quantum dot structures are a result ofthe ability of self-assembled GeSn by the Stranski-Krastanov (SK) orstrained layer epitaxy 1900 method that transitions from two dimensionalplanar growth 1901 to island growth 1902. In SK growth methodology alarger lattice constant semiconductor GeSn film 1903 is grown on asemiconductor with a smaller lattice constant Si 1904. Under properconditions two dimensional planar growth 1901 starts but quicklytransitions to island growth 1902 thus forming the GeSn QD 1905structure. Thus when a larger lattice constant semiconductor is grown ona semiconductor with a smaller lattice constant, when the criticalthickness of the larger lattice constant layer is exceeded QDs may form.The lattice mismatch between the two layers should be generally greaterthan 2% for dot formation. The lattice mismatch of GeSn to grown on Siwith a lattice constant of 5.43 Å, starts off at a 4% lattice mismatchfor low Sn % and can reach up to at 20% Sn content a 7% lattice mismatchwith Si. If SiGe layers are used as the barrier depending on the Gecontent in the SiGe the lattice mismatch could be significantly reduced.Typically quantum dots are less than 15 nm, but they can range from 1 to100 nm in size. Strained layer epitaxy 1900 is a methodology for growingquantum structures with dissimilar lattice constant materials. After the3D growth of the QD, the QD layer usually has a barrier layer grown ontop to finalize the quantum confinement.

FIG. 20 shows a flat band energy diagram of type II interband GeSn QDwith Si barriers 2000. The GeSn QD 2003 to Si 2002 heterojunction whichcan be of a type II heterojunction band alignment but at higher Sn % maybecome type I alignment. The figure shows a type II interband 2001transition from the conduction band of the Si 2002 to the GeSn QD holelevel 2004. The energy band gap is indicated in parenthesis below thematerial of interest. These type II interband 2001 transitions allow thepossibility of light emission. Type II energy band alignments allow foremission energy levels that can be the closest to the energy band gap ofthe bulk semiconductor. Note the size L_(QD) 2005 of the GeSn QD 2003determines the energy difference of the type II interband transition. Bychanging the size L_(QD) one can change the emission wavelength of thestructure.

FIG. 21 shows the possible range of emission wavelengths that areachievable in a type II GeSn quantum dot heterostructure with Sibarriers. The vertical axis is the wavelength (nm). The horizontal axisis the Quantum dot size (nm). This possible data represents the case forlow Sn % GeSn alloys. The graph shows that emission wavelengthsachievable depend on the size of the quantum dot. Note the wavelengthsare in the range of telecommunications. If the Sn % is increased thewavelengths achievable will get longer.

A type I heterostructure band alignment can occur for a GeSn QD if thesubstrate of interest is SiGe, because the addition of Ge to Sibarriers, altering the Si band structure. The addition of Ge into Siincreases the lattice constant, thus SiGe has a larger lattice constantthan Si. Thus it is possible to obtain higher Sn % GeSn QD on SiGebecause the SiGe lattice constant is larger than the Si latticeconstant.

FIG. 22 shows type I alignment of the GeSn QD with SiGe barriers 2200.The flat band energy diagram of GeSn QD 2203 with SiGe 2202 barrierswhich can be of type I alignment. The figure shows a type I interband2201 transition from the conduction band of GeSn QD electron level 2204to the GeSn QD hole level 2205. These type I interband 2201 transitionsallow the possibility of light emission. Note the size L_(QD) 2206 ofthe GeSn QD 2203 determines the interband transition energy. Because thearrangement is of a type I heterostructure the emission energies thatcan be achieved are typically higher than in a type II heterostructure.It is straightforward to calculate the range of wavelengths achievablein the GeSn quantum dot with SiGe barriers. Basically the emissionwavelengths that can be achieved for a type I alignment are shorter thanthat of type II QD heterostructures. By utilizing SiGe 2202 one canchange the Ge content of the SiGe thus changing the properties of theSiGe barrier layer to the GeSn QD 2203, thus in this structure theemission wavelengths achievable depends on the QD size L_(QD), the ratioof Ge to Sn in the GeSn 2203 material, and the ratio of Si to Ge in theSiGe 2202 barrier layer. If the Ge content is high enough (greater than50%) in the SiGe barrier it can be possible to grow GeSn in a planargrowth mode thus forming two dimensional growth or QWs.

The formation of QWs are more straightforward because the materials aregrown in a planar structure, and the QW can be coherently strained ornear latticed matched. FIG. 23 shows the methodology of planar growth2301 of the GeSn 2303 QW region on the GaAs 2302 bottom barrier layer.GaAs 2302 bottom barrier layer has a larger lattice constant then theGeSn 2303. But for low Sn % that GeSn 2203 can be grown coherently orpseudomorphic on the GaAs 2302. To finalize the QW layer a GaAs 2304 topbarrier layer is grown on the GeSn 2303 QW layer. Note that the barrierlayer generally has a larger bandgap energy then the QW layer. Thethickness of the QW region and the band alignment to the barriermaterials determine the allowable energy transitions.

FIG. 24 shows a flat band energy band diagram of a type I interband GeSnQW with GaAs barriers 2400. The band alignment of GeSn QW 2403 with GaAs2402 barrier can be of a type I heterojunction. Because the latticeconstant of GaAs and Ge is both about 5.66 Å, one could also use a Gebarrier material to obtain a type I alignment. Because for low Sn %GeSn, the GeSn growth on GaAs or Ge will be compressively strained. Aslong as the GeSn thickness is less than the critical thickness, coherentplanar or pseudomorphic growth can proceed on the GaAs or Ge barriermaterial, thus forming the GeSn QW. The Type I interband transitions arean excellent method for producing the emission of light fromsemiconductor heterostructures. The FIG. 24 shows a type I interband2401 transition from the conduction band GeSn QW electron level 2404 tothe GeSn QW hole level 2405. These type I interband 2401 transitionsallow the possibility of light emission. Note the size L_(C) 2406 of theGeSn QW 2403 determines the type I interband transition energy. Becausethe arrangement is of a type I heterostructure the emission energiesthat can be achieved are typically higher than in a type IIheterostructure. It is straightforward to calculate the range ofwavelengths achievable in the GeSn quantum dot with low Sn % with GaAsbarriers. Basically the emission wavelengths that can be achieved for atype I alignment are shorter than that of type II QD heterostructures.

For QW of a type I heterostructure the emission energies that can beachieved are typically higher than in a type II heterostructure. Type Iinterband transitions generally result in energy transitions that aregreater than the bulk GeSn transitions. Type II transitions can resultin energy transitions that can be less than the bulk GeSn transitions.Basically the emission wavelengths that can be achieved for a type Ialignment are shorter than that of type II QW heterostructures. As theSn % in GeSn increases the QW wavelengths will get longer.

By utilizing a direct energy gap GeSn bulk material or a GeSn quantumwell or a GeSn quantum dot in the base region of a transistor canachieve a light emitting HBT, that can emit light from 1000 nm to 5000nm.

For simplicity and clarity of illustration, the drawing figuresillustrate the general manner of construction, and descriptions anddetails of well-known features and techniques may be omitted to avoidunnecessarily obscuring the examples. Additionally, elements in thedrawing figures are not necessarily drawn to scale. For example, thedimensions of some of the elements in the figures may be exaggeratedrelative to other elements to help improve understanding of the examplespresented. Like reference numerals are used to designate like parts inthe accompanying drawings.

For an NPN HBT utilizing a GeSn base region certain requirements arenecessary to optimize the structure. FIG. 25 shows the general flat bandenergy diagram of an NPN GeSn double heterojunction bipolar transistor.This shows the general configuration of an optimized NPN HBT with GeSnbase 2500 region (energy band gap is indicated in parenthesis below thematerial of interest) can include an N-type emitter of material 1 E_(G1)2502, with a E_(G1) energy band gap greater than the GeSn 2501 (has anenergy band gap which is less than 0.66 eV) an forms the P-type baseregion; then a N-type collector material 2 E_(G2) 2503 where the energybandgap energy E_(G2) which may equal or be greater than the material 1E_(G1) 2502. Furthermore, the conduction band offset energies ΔE_(C1)2504 at the emitter base junction and the ΔE_(C2) 2505 at the basecollector junction should be zero or near zero in value. Smallconduction band offsets between the emitter-base junction and thecollector-base junction are desirable for electron transport.Additionally, the valence band offset ΔE_(V1) 2506 at the emitter basejunction should be as large as possible to ensure that there is no backinjection of holes from the GeSn 2501 P-type base to material 1 E_(G1)2502 N-type emitter material.

HBT performance can be improved, in some examples, by grading thecomposition of the base to decrease the energy band gap graduallythrough the base. The grading of the base energy band gap can create anelectric field, which causes a reduction in the transit time of thecharged carriers. The slope of Ge to GeSn compositional grade in thebase in this example can be varied from linear to discontinuousfunctions. The graded GeSn may comprise starting growth of Ge at theemitter than grading to higher Sn % GeSn at the collector or maycomprise starting growth of low Sn % GeSn at the emitter than grading tohigher Sn % GeSn at the collector.

FIG. 26 shows the general flat band energy diagram of an NPN HBT with agraded Ge—GeSn base 2600 region. Starting next to the N emitter material1 E_(G1) 2602 with a Ge or a low Sn % GeSn layer which is graded tohigher Sn % GeSn and is represented by Ge—GeSn 2601. This results in afield enhancement 2608 region. The general configuration of this NPN HBTwith the graded Ge—GeSn 2601 P-type base region can include an N-typeemitter of material 1 E_(G1) 2602, with a E_(G1) energy band gap greaterthan the Ge—GeSn 2601 energy band gap which is less than 0.66 eV; aGe—GeSn 2601 P-type base region; then a N-type collector material 2E_(G2) 2603 where the energy bandgap energy E_(G2) may equal or begreater than that of material 1 E_(G1) 2602. Furthermore, the conductionband offset energies ΔE_(C2) 2605 at the emitter base junction and theΔE_(C1) 2604 at the base collector junction should be zero or near zeroin value. Having small conduction band offsets between the emitter-basejunction and the collector based junction is desirable for electrontransport. Additionally, the valence band offset ΔE_(V1) 2606 at theemitter base junction should be as large as possible to ensure thatthere is no back injection of holes from the Ge—GeSn 2601 P-type base tomaterial 1 E_(G1) 2602 N-type emitter material.

The importance of the base region of the HBT can further be elucidatedby the following example. For a GaAs base HBT which has a base thicknessof 1000 Å, for an equivalent device the GeSn base in a HBT, the basethickness could be halved to 500 Å with no detrimental but enhancedresults. The F_(t) for GeSn HBT would increase because of the thinnerbase. Because the GeSn base resistivity (0.0002 ohm-cm) is 10 times lessthan GaAs resistivity (0.002) ohm-cm), the parameter F_(max) wouldincrease by a factor of (5*F_(t))^(1/2). This is because F_(t) increasedbecause the base thickness was halved, but the base sheet resistance ofthe GeSn base only increased by a factor of two, but it is still 5 timesless than the base sheet resistance of the GaAs HBT. Thus GeSn isexcellent material for high frequency HBT performance.

GeSn base material advantages, at low Sn % are similar properties to Ge.GeSn has a low band gap (lower than Ge: the term low energy band gapbase typically refers to the relevant semiconductors with band gaps lessthan 0.75 eV, like GeSn, Ge, InGaAs, GaAsSb) which results in lowturn-on voltage (less than 0.5 V) for the base emitter junction. GeSn(low Sn %<20%) hole mobility is high like Ge (2000 cm²/Vs as compared toGaAs 400 cm²/Vs) and acceptors can be incorporated to high density(>1×10¹⁹ cm⁻³), thus the base can be made ultra-thin (much less than 500Å) while maintaining a low base sheet resistance (P-type baseresistivity is about 0.0002 ohm-cm) which increases current gain anddecreases electron transit time. GeSn can be heavily doped P-type(>1×10¹⁹ cm⁻³). GeSn for low Sn concentration, has shallow acceptors, sothe hole concentration is generally equal to the acceptor doping leveland independent of temperature. The low base sheet resistance of GeSnresults in a high F_(max). The surface recombination velocity is low forP-type Ge and GeSn. GeSn can be made to become a direct gapsemiconductor at compositions in the range from 7% to 11% Sn contentthus useful as a light emitting semiconductor material.

A light emitting heterojunction bipolar transistor can be formed byplacing a GeSn quantum well or quantum dot in the base region of aheterojunction transistor. This is a methodology for the formation ofthe transistor laser.

FIG. 27 shows the resulting flatband energy diagram of an NPN HBT withGeSn QW or QD inserted in the P-type base region 2700 for the initialformation of a light emitting transistor. Material 3 E_(G3) 2703 is theP-type base region and forms the barrier to the GeSn QW or QD 2707 toget quantum confinement and also may serve as the waveguide material.The material 3 E_(G3) 2703 bandgap energy should be greater than theband gap energy of the bulk GeSn material which is less than equal to0.66 eV. The N-type emitter material 1 E_(G1) 2701 and N-type collectormaterial 2 E_(G2) 2702 may serve as the cladding layers of thetransistor laser. The general configuration of an optimized NPN HBT withcan include an N-type emitter of material 1 E_(G1) 2701, with an E_(G1)energy band gap greater than material 3 E_(G3) 2703 P-type base region.The N-type collector material 2 E_(G2) 2702 should have a bandgap energyE_(G2) equals to the material 1 E_(G1) 2701 or can be much larger. Theconduction band offset energies ΔE_(C1) 2704 at the emitter basejunction and the ΔE_(C2) 2705 at the base collector junction need not benear zero because a type I alignment assists in funneling the carriersinto the waveguide and to the GeSn QW or QD 2707. Additionally, thevalence band offset ΔE_(V1) 2706 at the emitter base junction should beas large as possible to ensure that there is no back injection of holesfrom the P-type base to material 1 E_(G1) 2701 N-type emitter material.

Additional variations could include grading of the quantum region in thebase material of such a device. FIG. 28 shows a slight variation to FIG.27 where the flat band energy diagram of NPN HBT has a GeSn QW or QD2807 inserted in the base region 2800 where the barrier layer that hasbeen graded 2808 from the P base material 3 2703.

Exemplary Configurations: note these are exemplary heterojunctionbipolar transistor and/or transistor laser configurations and are usedto illustrate the purposes and used of the various configurations. Invarious embodiments, the GeSn base region can be replaced by a graded Geto GeSn base region.

Exemplary Configuration 1, which is an NPN structure of a GaAsEmitter-GeSn Base-GaAs Collector symmetric double heterojunctiontransistor. Typically GaAs HBTs have been the standard of the industry.The device elucidated in this example can include a symmetric doubleheterojunction GaAs—GeSn—GaAs HBT device. This device can have desirablebase characteristics with a low voltage base turn-on (<0.66 eV dependingon Sn %, at 20% Sn % the band gap energy can be 0.25 eV) region and asymmetric double heterojunction thus eliminating the offset voltage inthe transistor output characteristic that reduces power addedefficiency.

FIG. 29 illustrates an exemplary flat band energy diagram of a GaAsEmitter-GeSn Base-GaAs Collector NPN symmetric double HBT 2900. Note thenear zero conduction band offset ΔE_(C)˜0.05 eV 2901 with a largevalence band offset ΔE_(V)˜0.72 eV 2902. Near zero conduction bandoffsets are generally less than 0.1 eV. This unique arrangement ofmaterials combines the high transconductance of heterojunction bipolartransistor (HBT) technology, and a desirable emitter-base heterojunction(wide band-gap GaAs 2904 emitter on a narrow band-gap high conductivityP⁺ GeSn 2903 base). The large valence band discontinuity between theGaAs emitter and GeSn base allows one to lightly N dope the GaAsemitter, while heavily doping P base GeSn 2903. The large valence banddiscontinuity prevents the back injection of holes into the emitter thuspreventing the degradation in the gain or beta of the transistor. TheGaAs 2905 collector provides a reasonable base collector breakdownbecause the band gap energy of the GaAs is 1.42 eV.

This unique arrangement of materials combines the high transconductanceof heterojunction bipolar transistor (HBT) technology, with thebreakdown voltage (using a GaAs collector), and a desirable emitter-baseheterojunction (wide band-gap GaAs emitter on a narrow band-gap highconductivity P-type GeSn base). The combination of a low band-gap (<0.66eV depending on Sn %) GeSn base coupled with a wide band-gap GaAs (canbe about 1.42 eV) collector can be used for high speed powerapplications. This symmetric double heterojunction bipolar transistordevice results in a minimum in the zero offset voltage in themeasurement of the collector current vs. the collector-emitter voltageas a function of the stepped voltage bias of the base-emitter junction,which is desirable for improving the power added efficiency of the NPNheterojunction bipolar transistors. The use of efficient GaAs—GeSn—GaAstransistors can significantly enhance battery life while also enablingoperation at high frequency response, which can be desirable when usedas RF power amplifiers for wireless or cellular phone applications.

FIG. 30 shows an exemplary cross-sectional device depiction ofembodiment of an NPN GaAs—GeSn—GaAs symmetric double HBT 3000. Thisdevice depiction shows the standard HBT in a mesa configuration. One cangrow this device epitaxially with a variety of techniques like MBE,MOCVD, etc. One starts with a high quality single crystalsemi-insulating GaAs substrate 3001. An N⁺ GaAs sub-collector 3002, isgrown first, followed by the N⁻ GaAs collector, 3003, then the GeSn Base3004 (which has been grown by MBE, MOCVD, PLD, etc.). An N⁻ GaAs emitter3005 is grown on the P-type base, followed by the N⁺ GaAs contact layer3006. Contact is made to the device through the emitter metal 3007, thebase metal 3008, and the collector metal 3009.

Additionally, the base can be compositionally graded from Ge—GeSn tohave field enhancement of the carriers. FIG. 31 illustrates an exemplaryflat band energy diagram of the NPN structure of a GaAs Emitter-gradedGe—GeSn 3101 Base-GaAs Collector double HBT 3000. FIG. 31 is a variationon FIG. 29, by including the graded Ge—GeSn 3101 P-type base region;this structure creates an electric field that accelerates the electronsacross the base to the collector, thus creating the field enhancement3102 region. The compositionally graded Ge—GeSn 2601 layer may compriseat the emitter a Ge or a low Sn % GeSn layer which is graded to higherSn % GeSn at the collector interface. The compositional grading rangecan go from Ge at the emitter to GeSn at various compositions up to 20%.

Table 1 shows an exemplary structure that could be grown for the NPNstructure of a GaAs Emitter-GeSn Base-GaAs Collector doubleheterojunction transistor. Note the table shows a GeSn base region or acompositionally graded Ge—GeSn base region, either which can be used inthe structure.

TABLE 1 Exemplary Epitaxial Structure of NPN GaAs—GeSn—GaAs HBT LayerLayer Name Description Comment 1 N⁺ cap ~1000 Å InGaAs (Te-doped >10¹⁹cm⁻³) Te = tellurium InGaAs layer is fully relaxed 2 N⁻ Emitter Cap~1500 Å GaAs (Si-doped ~5 × 10¹⁸ cm⁻³) Si = silicon 3 N⁻ Emitter ~500 ÅGaAs (Si-doped ~3 × 10¹⁷ cm⁻³) 4 P⁺ Base ~500 Å GeSn (B-doped >10¹⁹cm⁻³) Or graded Ge—GeSn 0 ≦ Sn % ≦ 20% Thickness range 100 Å-5000 Å 5 N⁻Collector ~10000 Å GaAs (Si-doped ~1 × 10¹⁶ cm⁻³) 6 N⁺ sub-collector~5000 Å GaAs (Si-doped ~5 × 10¹⁸ cm⁻³) 7 High Purity Buffer ~500 Å GaAs(undoped) UID 8 GaAs semi-insulating substrate Note the compositionallygraded Ge—GeSn layer may comprise at the emitter interface a Ge or a lowSn % GeSn layer which has a graded to higher Sn % GeSn at the collectorinterface. The grading range can go from Ge at the emitter to GeSn atvarious compositions up to 20%.

Exemplary GaAs advantages: The large valence band offset between GaAsemitter and GeSn base can stop back injection of holes into the emitter.This allows for low N-type doping of the emitter and high P-type dopingof the base, thus lowering base emitter capacitance while stillachieving sizable current gain. GeSn is near latticed matched to GaAs(˜5.65 Å), which enables dislocation free growth.

The GaAs—GeSn emitter base junction has a large valence (˜0.72 eV whichis larger than the GeSn band gap). This eliminates the back injection ofholes to the emitter from the base, which reduces the gain of thetransistor. Additionally, the base is doped heavily P-type(typically>1×10¹³ cm⁻³), with such high doping of the base, the emittervalence band offset blocks the holes even though the base doping is muchhigher than the N-type emitter doping (˜low 10¹⁷ cm⁻³). Furthermore,because GeSn has a low resistivity of 0.0002 ohm-cm, one can decreasethe thickness of the base significantly, while still moderatelyincreasing the base sheet resistance value. The frequency response ofthe device is related to the F_(t) and F_(max). The relationship betweentransit frequency F_(t) and the maximum oscillation frequency F_(max) isas follows for an HBT: F_(max)=(F_(t)/8πR_(B)C_(CB))^(1/2). The transitfrequency F_(t) is basically the inverse of the time for the electron totraverse the emitter, base and collector. The parameters R_(B) andC_(CB) refer to the base sheet resistance and the capacitance of thecollector base junction. The parameter F_(max) is the unity power gainfrequency and indicates the maximum frequency with power gain from adevice. The transit frequency can be further improved by having a highersaturation velocity for the collector.

It should be noted that there are many types of N-type and P-typedopants. For standard III-V semiconductors like GaAs, InP, InGaAs,InGaP, the N-type dopants are Si, Ge, Sn, Pb, S, Se, Te. The P-typedopants for standard III-V semiconductors are C, Zn, Be, Mg. Commondopants for group IV semiconductors like GeSn, Ge, Si, SiGe, GeSiSn forN-type dopants are P, As, Sb. The P-type dopants are B, Al, Ga.

Also designations such as N⁺ reference highly N-type doped material andN⁻ lightly doped N-type material. Also designations such as P⁺ referencehighly P-type doped material and P⁻ lightly doped P-type material.Unintentionally doped material can be denoted as UID.

In some examples, to fabricate a light emitting bipolar transistor mayrequire an insertion into the Ge base region (or GaAs base) a GeSnquantum dot or quantum well. FIG. 32 shows the exemplary flatband energydiagram of an NPN transistor laser structure with a GeSn QW or QD activeregion in a Ge P⁺ base/barrier material 3200. The Ge 3201 and 3202 formsthe P⁺ base and also acts as a barrier layer for quantum confine theelectrons and holes in the GeSn QW or QD 3203. QWs are formed by havinga large energy bandgap material surrounded by a low energy bandgapmaterial which results in two dimensional electron confinement. For a QDthe growth of a large lattice constant material on a smaller latticeconstant material results in strained layer epitaxy allowing theself-assembled three dimensional island growth. Typical thickness ofquantum wells can be about 10 nm but they can range from 1 nm to 50 nmdepending on the wavelength of interest that needs to be produced.Typical quantum dot diameters are in the range of 1 nm-20 nm, but aredependent on the wavelength of light that needs to be emitted. The GeSnQW or QD 3203 inserted into a Ge 3201 & 3202 base/barrier serves for thecollection region for electrons and holes to recombine to generatelight. The Ge 3201 & 3202 also serve as the optical confinement layerand the waveguide material. The GaAs 3204 & 3205 serves as theemitter/cladding and collector/cladding material for this structure. Thecladding serves as funneling carriers into the active/waveguide regionand traps the emit light in the waveguide structure. The large energybandgap difference between the GaAs 3204 & 3205 and the Ge 3201 & 3202ensures a large index of refraction difference at the N⁻emitter/cladding 3206 and P⁺ base/barrier 3207 junction and a largeindex refraction difference at the P⁺ base/barrier 3208 and N⁻collector/cladding 3209, thus making an excellent waveguide 3210 tooptically confine the light produced by the active region.

FIG. 33 shows a possible cross sectional device depiction of an NPNtransistor laser structure with a GeSn QW or QD 3306 active region in aGe P-type base/barrier material 3300. The structure can be grown on N⁺GaAs conducting substrate 3303, which is the seed crystal to grow thefull structure. N GaAs collector/cladding 3304 and the N⁻ GaAsemitter/cladding 3308 do dual functions of optical confinement of thelight 3309 produced and the controlling the flow of electrons and holes.The P⁺ Ge Base 3305 and 3307 form the barrier material for the GeSn QWor QD 3306, and also provide the waveguide material. The laser canrequire a resonant cavity to get optical gain, and typically this canformed from the front cleaved facets 3302 and back cleaved facets 3301of the semiconductor crystalline structure.

Table 2 shows an exemplary epitaxial structure of an NPN light emittingwith a GeSn QW or QD active region in a Ge P-type base/barrier HBT.

TABLE 2 Exemplary Epitaxial Structure of NPN light emitting with a GeSnQW or QD active region in a Ge P-type base/barrier HBT Layer Layer NameDescription Comment 1 N⁺ cap ~1000 Å InGaAs (Te-doped >10¹⁹ cm⁻³) 2 N⁻Emitter/Cladding ~1000 Å GaAs (Si-doped ~3 × 10¹⁷ cm⁻³) 3 P⁺Base/Barrier ~450 Å Ge (B-doped >10¹⁹ cm⁻³) (B = boron) 4 QW undoped~100 Å GeSn for light emission Or QD Sn content can be: 0 ≦ Sn % ≦ 20%1000 nm-4000 nm UID QW (Sn % ~7% to 12% GeSn) QW thickness range 10Å-1000 Å QD (Sn % ~12% to 20% GeSn) QD size range 10 Å-200 Å 5 P⁺Base/Barrier ~450 Å Ge (B-doped >10¹⁹ cm⁻³) 6 N⁻ Collector ~1000 Å GaAs(Si-doped ~1 × 10¹⁶ cm⁻³) 7 N⁺ sub-collector ~500 Å GaAs (Si-doped ~5 ×10¹⁸ cm⁻³) 8 N⁺ Buffer ~500 Å GaAs (Si-doped ~5 × 10¹⁸ cm⁻³) GaAs N⁺conducting substrate Crystalline

The semiconductor alloy In_(0.49)Ga_(0.51)P (InGaP) can be latticematched to GaAs. InGaP can be grown in a dis-ordered phase, orderedphase, or a combination of the two. The disordered InGaP phase has abandgap energy of 1.9 eV. The band gap of the ordered InGaP is about1.85 eV. In some examples, to fabricate a light emitting bipolartransistor may require an insertion into the GaAs base region a GeSnquantum dot or quantum wire, or quantum well layer. The GaAs is the ptype base material but also acts as a barrier layer to quantum confinethe electrons and holes in the GeSn QW. QWs are formed by having a largeenergy bandgap material surrounded by a low energy bandgap materialwhich results in two dimensional electron confinement. For a QD thestrained layer growth results in three dimensional electron confinement.Typical thicknesses of quantum wells are about 100 Å but they could belarger or less than that thickness depending on the emission wavelengthdesired. Typical quantum dot diameters are in the range of 1 nm-20 nm,but are dependent on the wavelength of light that needs to be emitted.

FIG. 34 shows the exemplary flatband energy diagram of an NPN transistorlaser structure with a GeSn QW or QD active region in a GaAs P⁺base/barrier material 3400. The In_(0.49)Ga_(0.51)P disordered 3401 &3405 serves as the N⁻ emitter/cladding and N⁻ collector/cladding layers.Note the relevant band discontinuities ΔE_(C), ΔE_(V) are shown in thefigure. The laser includes a GeSn QW or QD 3403 active region for thecollection region for electrons and holes to recombine to generatelight, which is inserted into a P⁺ GaAs base/barrier 3402 & 3404, whichalso serves as the barrier layer of the GeSn QW or QD 3403 activeregion, thus serving also as the optical waveguide 3410 material. Thelarge energy bandgap difference between the In_(0.49)Ga_(0.51)Pdisordered 3401 & 3405 and the GaAs base/barrier 3402 & 3404 ensures alarge index of refraction difference at the N⁻ emitter/cladding 3406 andP⁺ base/barrier 3407 junction and a large index refraction difference atthe P⁺ base/barrier 3408 and N⁻ collector/cladding 3409, thus making anexcellent waveguide 3410 to optically confine the light produced by theactive region.

FIG. 35 shows a possible cross-sectional device depiction of an NPN edgeemitting transistor laser structure with a GeSn QW or QD active regionin a GaAs P⁺ base/barrier material 3500. A GeSn QW or QD 3506 activeregion in a P⁺ GaAs base 3505 & 3507 which also functions as the barriermaterial for the QW or QD. This is an exceptional device because thetype I discontinuities form an excellent optical and electricalconfining structure. The structure can be grown on N⁺ GaAs conductingsubstrate 3503, which is the seed crystal to grow the full structure. N⁻InGaP (disordered) collector/cladding 3504 and the N⁻ InGaP (disordered)emitter/cladding 3508 do dual functions of optical confinement of thelight 3509 produced and the controlling the flow of electrons and holes.P⁺ GaAs base 3505 & 3507 form the barrier material for GeSn QW or QD3506, and also provide the waveguide material. The laser can require aresonant cavity to get optical gain, and typically this can formed fromthe front cleaved facets 3502 and back cleaved facets 3501 of thesemiconductor crystalline structure.

Table 3 shows an exemplary table of the epitaxial structure of an NPNlight emitting GeSn QW or QD active region in a GaAs P-type base/barrierHBT.

TABLE 3 An exemplary epitaxial structure of an NPN edge emittingtransistor laser structure with a GeSn QW or QD active region in a GaAsP-type base/barrier material Layer Layer Name Description Comment 1 N⁺cap ~1000 Å InGaAs (Te-doped >10¹⁹ cm⁻³) 2 N⁻ Emitter/Cladding ~5000 ÅIn_(0.49)Ga_(0.51)P (Si-doped ~3 × 10¹⁷ cm⁻³) Disordered 3 P⁺Base/Barrier ~500 Å GaAs (B-doped >10¹⁹ cm⁻³) 4 Undoped QW or QD ~55 ÅGeSn for light emission Sn content can be: 0 ≦ Sn % ≦ 20% 1000 nm-4000nm QW (Sn % ~7% to 12% GeSn) QW thickness range 10 Å-1000 Å QD (Sn %~12% to 20% GeSn) QD size range 10 Å-200 Å 5 P⁺ Base/Barrier ~500 Å GaAs(B-doped >10¹⁹ cm⁻³) 6 N⁻ Collector/Cladding ~5000 Å In_(0.49)Ga_(0.51)P(Si-doped ~3 × 10¹⁷ cm⁻³) Disordered 7 N⁺ sub-collector ~500 Å GaAs(Si-doped ~5 × 10¹⁸ cm⁻³) 8 N⁺ Buffer ~500 Å GaAs (Si doped ~5 × 10¹⁸cm⁻³) 9 N⁺ GaAs conducting substrate Crystalline

The transistor laser has the integrated features of both the transistorand the laser. From such a structure it can be straightforward tosimplify the structure and grow a separate confinement heterostructure(SCH) laser.

FIG. 36 shows an exemplary flat band energy diagram of an SCH laserutilizing a GeSn QW or QD region located in UID Ge barrier/OCL layer3602 & 3604 region with P⁺ GaAs 3601 cladding and N⁺ GaAs 3605 claddinglayers. This structure represents a PN junction or diode with anunintentionally doped (UID) active region and optical confinement regionbetween the P⁺ GaAs 3601 and N⁺ GaAs 3605 cladding. The UID Ge 3602 and3604 forms the barrier material for the GeSn QW or QD 3603 activeregion. The combination of the barrier and active region forms thewaveguide 3606 of the laser. The P⁺ GaAs 3601 cladding region serves forinjection of the holes and for the optical confinement of the lightemitted from the active region. The N⁺ GaAs 3605 cladding region servesfor injection of the electrons and for the optical confinement of thelight emitted from the active region. Though this depicts a symmetricstructure it can be also asymmetric.

FIG. 37 shows a cross sectional depiction of a SCH ridge laser utilizinga GeSn QW or QD region located in UID Ge barrier/OCL region with P-typeGaAs and N-type GaAs cladding. The structure can be grown on N⁺ GaAsconducting substrate 3703. N⁺ GaAs cladding 3704 serves for theinjection of electrons into the active region and the bottom claddingfor the optical confinement of the light emitted from the active region.The P⁺ GaAs cladding 3708 serves for the injection of holes into theactive region and, additionally, the top cladding for opticalconfinement of the light 3709 emitted from the active region. The UID Ge3705 & 3707 forms the barrier material for the GeSn QW or QD 3706, andalso provide the OCL material. The laser can require a resonant cavityto get optical gain, and typically this can be formed from the frontcleaved facets 3702 and back cleaved facets 3701 of the semiconductorcrystalline structure. The ridge structure provides the vertical guidingof the current into the active region.

Table 4 shows an exemplary epitaxial structure SCH injection diode laserwith a GeSn QW or QD region with Ge barriers.

TABLE 4 Exemplary epitaxial structure SCH injection diode laser with aGeSn QW or QD region with Ge barriers. Layer Layer Name DescriptionComment 1 P⁺ cap ~1000 Å GaAs (Zn doped >10¹⁹ cm⁻³) 2 P⁺ Cladding ~10000Å GaAs (Zn doped ~1 × 10¹⁸ cm⁻³) 3 Barrier/OCL ~450 Å Ge UID 4 QWundoped ~100 Å GeSn for light emission Or QD Sn content can be: 0 ≦ Sn %≦ 20% 1000 nm-4000 nm QW (Sn % ~7% to 12% GeSn) QW thickness range 10Å-1000 Å QD (Sn % ~12% to 20% GeSn) QD size range 10 Å-200 Å 5Barrier/OCL ~450 Å Ge UID 6 N⁺ Cladding ~10000 Å GaAs (Si-doped ~1 ×10¹⁸ cm⁻³) 8 N⁺ Buffer ~5000 Å GaAs (Si-doped ~5 × 10¹⁸ cm⁻³) 9 GaAs N⁺conducting substrate Crystalline

A variation of the laser structure could incorporate GeSn QW or QDregion in a UID GaAs barrier/waveguide region, and utilizing latticedmatch InGaP as the cladding material. FIG. 38 shows an exemplary flatband energy diagram of an SCH diode laser utilizing a GeSn QW or QDregion located in UID GaAs 3802 & 3804 barrier/OCL region with P⁺ InGaP3801 disordered and N⁺ InGaP 3805 disordered cladding. This structurerepresents a PN junction or diode with an unintentionally doped (UID)active region and optical confinement region between the P⁺ InGaP 3801disordered and N⁺ InGaP 3805 disordered cladding. The UID GaAs 3802 &3804 forms the barrier material for the GeSn QW or QD 3803 activeregion. The combination of the barrier and active region forms thewaveguide 3806 of the laser. The P⁺ In_(0.49)Ga_(0.51)P disordered 3801top cladding region serves for injection of the holes and for theoptical confinement of the light emitted from the active region. The N⁺In_(0.49)Ga_(0.51)P disordered 3805 bottom cladding region serves forinjection of the electrons and for the optical confinement of the lightemitted from the active region. Though this depicts a symmetricstructure it can be also asymmetric.

Table 5 shows an exemplary epitaxial structure SCH injection diode laserwith a GeSn QW or QD region with GaAs barriers.

TABLE 5 Exemplary epitaxial structure SCH injection diode laser with aGeSn QW or QD region with GaAs barriers. Layer Layer Name DescriptionComment 1 P⁺ cap ~1000 Å GaAs (Zn doped >10¹⁹ cm⁻³) 2 P⁺ Cladding ~10000Å In_(0.49)Ga_(0.51)P (Zn-doped ~1 × 10¹⁸ cm⁻³) Disordered 3 Barrier/OCL~450 Å GaAs UID 4 QW undoped ~100 Å GeSn for light emission Or QD Sncontent can be: 0 ≦ Sn % ≦ 20% 1000 nm-3000 nm QW (Sn % ~7% to 12% GeSn)QW thickness range 10 Å-1000 Å QD (Sn % ~12% to 20% GeSn) QD size range10 Å-200 Å 5 Barrier/OCL ~450 Å GaAs UID 6 N⁺ Cladding ~10000 ÅIn_(0.49)Ga_(0.51)P (Si-doped ~1 × 10¹⁸ cm⁻³) Disordered 8 N⁺ Buffer~5000 Å GaAs (Si-doped ~5 × 10¹⁸ cm⁻³) 9 GaAs N⁺ conducting substrateCrystalline

The lattice constant of GaAs and Ge is both about 5.65 Å. The GeSnlattice constant can change from 5.66 Å to 5.833 Å at about 20% Sncontent, the range of lattice mismatch at the highest Sn content isabout 3%. This makes GeSn useful for growth on Ge or GaAssemiconductors, because at low Sn % GeSn can be grown coherentlystrained or psuedomorphic on either Ge or GaAs. For low Sn % GeSn thelattice mismatch is reasonable and films can be grown pseudomorphic(strained) if thin enough, or partial relaxation may occur for thickerfilms (1000 Å or more). Thus for growth of GeSn QW on Ge or GaAs, planargrowth can be achieved. FIG. 39 shows the planar growth of strained GeSn3901 (low Sn %) on Ge, with Ge barriers above and below the QW GeSnfilm. For low Sn % GeSn both Ge or GaAs barriers can be used for theformation of the strained GeSn QW.

The formation of quantum dot structures are a result of the ability ofself-assembled GeSn quantum dots by the Stranski-Krastanov (SK) methodthat transitions from two dimensional to island growth. The diagrambelow shows the methodology of formation. A larger lattice constantsemiconductor is grown on a semiconductor with a smaller latticeconstant. Under proper conditions two dimensional planar growth startsbut quickly transitions to island growth thus forming the quantum dotstructure. Thus the lattice constant of GeSn is typically greater thanthat of Ge. Typically the formation of the quantum dots occur when thecritical thickness of the GeSn layer is exceeded. The lattice mismatchshould be typically greater than 2% for dot formation. The range oflattice mismatch of GeSn to that of Ge (or GaAs) is approximately startsat 0% and goes up to 3% lattice mismatch at 20% Sn in GeSn. FIG. 40shows the island growth of strained GeSn 4001 (high Sn %) on Ge barrierlayer with the subsequent formation of the QD layer.

GeSn quantum structure provide a unique methodology to form both QW andQD in the same structure because the Sn % for becoming a direct gapsemiconductor can vary from 7% Sn % 20%. GeSn is indirect up to about 7%Sn then becomes a direct gap semiconductor with an energy bandgap of0.585 eV and a lattice constant of about 5.725 Å, and the band gapenergy typically reduces to about 0.25 eV at 20% Sn with a latticeconstant of about 5.835 Å. Utilizing Ge or GaAs barriers which have alattice constant of about 5.65-5.66, one can calculate the latticemismatch at various compositions. The lattice mismatch between GeSn toGaAs or Ge at 7% Sn content GeSn is about 1%. The lattice mismatchbetween GeSn to GaAs or Ge at 20% Sn content in GeSn is about 3%.Typically the formation of the quantum dots generally occur when thecritical thickness of the GeSn layer is exceeded. The lattice mismatchshould be typically greater than 2% for quantum dot formation. A 2%lattice mismatch of Ge or GaAs to the GeSn corresponds to a latticeconstant of about 5.76 Å which is about 12% Sn in GeSn. Thus if onegrows on Ge or GaAs barriers, one can get GeSn planar direct gap Type IQW for 7% Sn %≦12% and direct gap type I QD for 12%≦Sn %≦20%. The GeSnenergies vary from 7% Sn with a bandgap energy of 0.585 eV; to 12% Snwith a bandgap energy of 0.48 eV; to 20% Sn with a bandgap energy ofabout 0.25 eV. Thus GeSn QW energies could be in the near-IR and theGeSn QD energies could be in the mid-IR, utilizing the exact same laseror transistor laser structure.

Exemplary Configuration 2A: Ordered InGaP Emitter-GeSn Base-GaAsCollector double heterojunction transistor. The device elucidated caninclude a double heterojunction InGaP—GeSn—GaAs HBT device. InGaP at thecomposition In_(0.49)Ga_(0.51)P is latticed matched to GaAs and is adirect gap semiconductor. In_(0.49)Ga_(0.51)P can be grown in two formsordered and disordered. InGaP semiconductor grown by various epitaxialgrowth technologies can be latticed matched to GaAs. At high temperaturegrowth the InGaP can grow in a crystalline structure such that thesheets of In—P and Ga—P atoms can alternate in the (001) planes of theface centered cubic (FCC) unit cell without the intermixing of the Gaand In atoms on the lattice planes. This results in an almost zeroconduction band discontinuity between the InGaP and GaAs and is calledthe ordered phase (this can be of weakly type I or weakly type IIbecause it is close to zero). The ordered phase has a band gap energy of1.85 eV and the dis-ordered phase has a bandgap energy of 1.9 eV, thusthe ordered phase is about bandgap energy is about 0.05 eV less than thedisordered phase. With different growth conditions, the In and Ga atomscan intermix and the disordered InGaP phase can form, which has a largerconduction band offset of 0.1 eV (type I) vs. 0.03 eV for the orderedphase. It is also possible to have a mixture of ordered and disorderedInGaP materials. The lattice constant of GaÅs and Ge is about 5.66 Å,and this is also the lattice constant of InGaP at the compositionIn_(0.49)Ga_(0.51) P.

In some examples, the ordered phase may have an advantage to thedisordered phase, because the ordered phase has near zero conductionband offset. In some examples, this device has desirable basecharacteristics with a low voltage base turn-on region and that the GeSnbase region can be directly inserted into a standard InGaP—GaAs HBTs,which is typically used in RF power amplifiers in cellular handsets tosend the voice and data to the cell tower. Additionally, in an invertedHBT structure by using the ternary alloy InGaP as the emitter andvarying the In composition away from the latticed matched condition,strain can be introduced into the GeSn base layer, thus the GeSn layercan be tensile or compressively strained.

FIG. 41 shows an exemplary flat band energy diagram of an NPN HBT withan ordered InGaP emitter, GeSn base and a GaAs collector. This type ofHBT structure is called an asymmetric double heterojunction device. Noteconduction band offsets are less than 0.1 eV and are in the near zeroconduction band offset range and are desirable for NPN transistors. Theordered In_(0.49)Ga_(0.51)P 4101 (disordered InGaP can also be usedhere) is an excellent material for the emitter because it has a smallconduction band offset and a large valence band offset with GeSn 4102 P⁺base region. The N-type GaAs 4103 is a proven collector material forHBTs

Additionally, the base can be graded from Ge—GeSn 4201 to have electricfield enhancement of the charge carriers (electrons). Such structurecreates an electric field that accelerates the electrons across the baseto the collector. FIG. 42 shows an exemplary flat band energy diagram ofan NPN HBT with an ordered InGaP N emitter, a graded Ge—GeSn 4201 P⁺base and a GaAs N collector. The graded Ge—GeSn 4201 layer may compriseat the emitter a Ge or a low Sn % GeSn layer which is graded to higherSn % GeSn at the collector interface. The grading range can go from Geat the emitter to GeSn at various compositions up to 20%. The grading ofthe base provide for a field enhancement 4202 region. One could grow theemitter first and then grade the Ge to GeSn.

Table 6 shows an exemplary epitaxial structure of an NPN HBT with anordered InGaP emitter, GeSn base region and a GaAs collector.

TABLE 6 Epitaxial structure of NPN HBT with ordered InGaP emitter, GeSnbase and GaAs collector. Layer Layer Name Description Comment 1 N⁺ cap~1000 Å InGaAs (Te-doped >10¹⁹ cm⁻³) 2 N⁻ Emitter Cap ~1500 Å GaAs(Si-doped ~5 × 10¹⁸ cm⁻³) 3 N⁻ Emitter ~500 Å In_(0.49)Ga_(0.51)P(Si-doped ~3 × 10¹⁷ cm⁻³) ordered 4 P⁺ Base ~500 Å GeSn (B-doped >10¹⁹cm⁻³) Or graded Ge—GeSn 0 ≦ Sn % ≦ 20% Thickness range 100 Å-5000 Å 5 N⁻Collector ~10000 Å GaAs (Si-doped ~1 × 10¹⁶ cm⁻³) 6 N⁺ sub-collector~5000 Å GaAs (Si-doped ~5 × 10^(18 cm) ³) 7 High Purity Buffer ~500 ÅGaAs (un-doped) UID 8 GaAs semi-insulating substrate Crystalline

It should be noted that there are many types of N-type and P-typedopants. For standard III-V semiconductors like GaAs, InP, InGaAs,InGaP, the N-type dopants are Si, Ge, Sn, Pb, S, Se, Te. The P-typedopants for standard III-V semiconductors are C, Zn, Be, Mg. Commondopants for group IV semiconductors like GeSn, Ge, Si, SiGe, GeSiSn forN-type dopants are P, As, Sb. The P-type dopants are B, Al, Ga.

Ge used as a base material in HBTs has many advantages, Ge has thefollowing properties that make it an excellent P-type base. Ge has a lowband gap (the term low energy band gap base typically refers to therelevant semiconductors with band gaps less than 0.75 eV, like GeSn, Ge,InGaAs, GaAsSb) which results in a low turn-on voltage. The Ge holemobility is high (2000 cm²/Vs) and acceptors can be incorporated to highdensity (>1×10¹⁹ cm⁻³), thus the base can be made ultra-thin (less than5000 Å) while maintaining a low base sheet resistance (<<18 Ohm/sq)which increases current gain and decreases electron transit time. Ge hastrue shallow acceptors, so that the hole concentration is generallyequal to the acceptor doping level and independent of temperature. Gehas a low base resistivity (0.0002 ohm-cm) which results in a highF_(max). The surface recombination velocity is low for P-type Ge. lowresistance ohmic contacts can be formed on P-type Ge. Ge mobility(electron and hole) can be significantly improved by being biaxiallytensile strain, thus for both NPN and PNP structures the base sheetresistance can be improved significantly. The Ge hole mobility undertension can exceed 10,000 cm²/Vs and under biaxial compression of Geenhances the hole mobility but degrades the electron mobility. Thus Geis a desirable base material thus GeSn will have similar properties

Typically for HBTs the collector is grown first followed by growing thebase and then growing the emitter. However, for this structure, it canbe advantageous to grow an inverted HBT. By growing theIn_(0.49)Ga_(0.51)P emitter first, one could increase the indium contentto greater than 49% In, thus, the bandgap energy would be reduced, butthe lattice constant would be increased. The conduction band offsetbetween the InGaP (In %>49) would cause the bands to be closer to zerooffset between the InGaP and Ge. This methodology could be applied to aGe base. FIG. 43 shows an exemplary flat band energy diagram of aninverted NPN HBT structure where the emitter is grown first, and thebase material is tensile strained Ge. This Inverted HBT structureemitter grown first collector up 4300 has a field enhancement region4303 in the compositionally graded InGaP 4301 N⁻ emitter. This structureallows for the P⁺ base to be tensile strained Ge 4302, thus enhancingthe hole mobility. Additionally, one could decrease the In % in theInGaP and then the Ge would be biaxially compressively strained.

The device structure has the following advantages, by growing theemitter InGaP on GaAs, one can initially lattice match the InGaP to theGaAs as shown. When the In composition can be increased to the pointwhere Ge is biaxially tensile strained to about at about 1.75% it thenbecomes a direct gap semiconductor.

Tensile Strain effects on Ge: It has shown that biaxial tensilecompression causes enhancements in the hole and electron mobility.Biaxial tension on the band structure of Ge breaks the heavy hole andlight hole band degeneracy and raises the light hole above the heavyhole band. This effectively increases the hole mobility. Typically theGe band structure shows that it is an indirect semiconductor because the“L” point <111> is the conduction band minimum and the gamma “Γ” pointis the valence band maximum. However Ge becomes direct gap semiconductorwith 1.4% biaxial tensile strain or greater, because the gamma “Γ” pointin Ge band structure gets closer to the valence band maximum faster thanthe L point <111>, thus making it a direct band gap semiconductor.

With biaxial tensile strain a dramatic increase in the Ge hole mobility“μ_(h)”. Thus, by growing an inverted emitter structure one caneffectively tensile or compressive strain the Ge (or GeSn) layer with nodegradation in performance. It has been shown experimentally thatbiaxial tensile can increase the in-plane hole mobility at 3% biaxialstrain of a Ge hole mobility>40,000 cm²/Vs. If the InGaP is graded tohigher In % then an electric field can be built in that can promote thefree charge carriers from the emitter into the base region.

Compressive strain effects on Ge: Ge under biaxial compression shows anenhancement in the in-plane hole mobility. It has been shownexperimentally that biaxial compressive strain of 1.7% in the Ge layerincreases the low field hole mobility by a factor of 3.38 to 6350cm²/V-s.

The elucidated tensile and compressive strain effects should also workfor GeSn for low compositions of Sn %. FIG. 44 shows an exemplaryflatband energy diagram showing an inverted tensile strained GeSn HBTstructure emitter grown first collector up 4400. This NPN HBT structurewhere the emitter is grown first, and the base material is tensilestrained GeSn 4402 has some advantages. This inverted HBT structure hasa field enhancement region 4303 in the graded InGaP 4301 N⁻ emitter.This structure allows for the P⁺ base to be tensile strained GeSn 4402,thus enhancing the hole mobility. By utilizing InGaP in thisconfiguration one can strain the GeSn in tension or compression.

If an inverted structure is not desirable, the collector grown firststructure (emitter up) can be grown and the InGaP collector can begraded as follows; from the GaAs sub-collector the InGaP starts at 49%In, then is slowly graded up to an In % greater than 49% at the start ofthe GeSn base region. This will result in field enhancement region inthe collector to accelerate the electrons to the sub-collector. FIG. 45shows an exemplary flatband energy diagram showing in an NPNconfiguration a compressively strained GeSn HBT collector grown firstemitter up 4500 structure, where the compressive strained GeSn 4501 isgrown on the N⁻ collector InGaP 4502 compositionally graded from In 49%to >49% collector which now has a field enhancement region 4503. Thisdevice has the advantage that it is a true double heterojunction almostsymmetric device. This would minimize the offset voltage found instandard InGaP—GaAs HBTs that causes a reduction in power addedefficiency. Note that compressively strained Ge could also be used asthe base region in this device.

An additional variation of this device results in GeSn that may or maynot be biaxially strained, by having both the emitter and collectorInGaP layers compositionally graded. Basically this is a combination ofthe previously described embodiments for the strained GeSn HBTs. Herethe emitter and collector both have field enhancement regions becausethe InGaP is graded in both layers. The standard configuration where thecollector is grown first (emitter up) and the InGaP collector can begraded as follows; from the GaAs sub-collector the InGaP starts at 49%In, then is slowly graded up to an In % greater than 49% at the start ofthe GeSn base. This process is repeated for the emitter where InGaPstarts at 49% In, then is slowly graded up to an In % greater than 49%at the start of N emitter contact region. FIG. 46 shows an exemplaryflatband energy diagram showing a GeSn Double HBT Structure gradedEmitter and graded Collector grown first 4600, where the GeSn 4605 basemay or may not be compressively strained. The N⁻ emitter InGaP 4601 isgraded from In % 49% to >49% has a field enhancement region 4602. The N⁻collector InGaP 4603 is graded from In % 49% to <49% has a fieldenhancement region 4604. The field enhancement regions cause anacceleration of the electrons to the NPN device, thus reducing thetransit time of the device.

Note through this patent InGaP is used throughout the text. Whereordered InGaP is referred to an equivalent device using disordered InGaPcan be used. Likewise where disordered InGaP is used ordered InGaP alsobe used. Though at some instance the lattice matched composition to GaAsis used In_(0.49)Ga_(0.51)P. Though this is a useful reference numberfor starting the InGaP layer can be graded or have a differentcomposition.

Exemplary Configuration 2B: NPN Disordered InGaP Emitter-GeSn Base-GaAsCollector double heterojunction transistor: The device elucidated caninclude a double heterojunction disordered In_(0.49)Ga_(0.51)P—GeSn—GaAsHBT device. In some examples, this device has desirable basecharacteristics with a low voltage base turn-on region. This devicestructure can be directly inserted into standard manufacturingInGaP—GaAs HBT. This is a slight variation on configuration 2A. Thedifference is the conduction band offset of disordered InGaP to GeSn isabout 0.15 eV (ordered InGaP was about 0.08 at its low Sn %), and thevalence band offset of the disordered InGaP to GeSn (low Sn %) is 1.09eV (ordered InGaP was about 1.11 eV at low Sn %). In some instances itcan be easier to grow the disordered InGaP.

Exemplary Configuration (2C) NPN AlGaAs Emitter-GeSn Base-GaAs Collectordouble heterojunction transistor: The device elucidated can include adouble heterojunction AlGaAs—GeSn—GaAs HBT device. In some examples,this device has desirable base characteristics with a low voltage baseturn-on region. This is a second variation on configuration 2A. ForAlGaAs for Al % less than 0.4 the material is direct gap semiconductor.The energy band gap of Al_(0.3)Ga_(0.7)As, a typical emittercomposition, is 1.8 eV (direct gap) as opposed In_(0.49)Ga_(0.51)P,which has an energy band gap of 1.85 eV (direct gap). The difference isthe conduction band offset of Al_(0.3)Ga_(0.7)As to GeSn (low Sn %) is0.29 eV (ordered InGaP was 0.08), and the valence band offset ofAl_(0.3)Ga_(0.7)As to GeSn (at low Sn %) is about 0.85 eV (ordered InGaPwas 1.11 eV). FIG. 47 shows an exemplary flatband energy diagram of theNPN AlGaAs Emitter-GeSn Base-GaAs Collector double HBT 4700. In this HBTthe N emitter is Al_(0.3)Ga_(0.7)As 4701 but the Al % could be varied todifferent levels for optimized transport.

To fabricate a light emitting NPN heterojunction bipolar transistor inthis Al_(0.3)Ga_(0.7)As (InGaP could also be used as the emitter andcollector) emitter HBT, a GeSn QW or QD region can be inserted into theGaAs base (Ge could also be used as the base). FIG. 48 shows anexemplary flat band energy diagram of the NPN HBT laser with AlGaAsemitter/cladding and AlGaAs collector/cladding 4800. This is anexceptional device because the type I alignment allows for excellentcarrier and optical confinement. The device structure is symmetric. TheN⁻ Al_(0.3)Ga_(0.7)As 4801 & 4805 is an excellent cladding layer and haslittle mismatch with GaAs. The P⁺ GaAs base 4802 & 4804 acts as thebarrier layer for the GeSn QW or QD 4803 active region. The large energybandgap difference between the N⁻ Al_(0.3)Ga_(0.7)As 4801 & 4805 and theP⁺ GaAs base 4802 & 4804 ensures a large index of refraction differenceat the N⁻ emitter/cladding 4807 and P⁺ GaAs 4802 base interface; and alarge index refraction difference at the P⁺ GaAs base 4804 and N⁻collector/cladding 4808, thus making an excellent waveguide 4806 tooptically confine the light produced by the active region. The laser canrequire a resonant cavity to get optical gain, and typically this canformed from the front and back cleaved facets of the semiconductorcrystal wafer.

Table 7 shows an exemplary table of the epitaxial structure of an NPNlight emitting AlGaAs—GaAs—GeSn—GaAs—AlGaAs HBT.

TABLE 7 An exemplary epitaxial structure of an NPN edge emittingtransistor laser structure with a GeSn QW or QD active region in a GaAsP-type base/barrier material and AlGaAs Cladding. Layer Layer NameDescription Comment 1 N⁺ cap ~1000 Å InGaAs (Te-doped >10¹⁹ cm⁻³) 2 N⁻Emitter/Cladding ~5000 Å Al_(0.3)Ga_(0.7)As (Si-doped ~3 × 10¹⁷ cm⁻³)Diffferent A % AlGaAs can be used 3 P⁺ Base/Barrier ~500 Å GaAs (B-doped>10¹⁹ cm⁻³) 4 Undoped QW or QD ~55 Å GeSn for light emission Sn contentcan be: 0 ≦ Sn % ≦ 20% 1000 nm-4000 nm QW (Sn % ~7% to 12% GeSn) QWthickness range 10 Å-1000 Å QD (Sn % ~12% to 20% GeSn) QD size range 10Å-200 Å 5 P⁺ Base/Barrier ~500 Å GaAs (B-doped >10¹⁹ cm⁻³) 6 N⁻Collector/Cladding 5000 Å Al_(0.3)Ga_(0.7)As (Si-doped ~3 × 10¹⁷ cm⁻³)Diffferent A % AlGaAs can be used 7 N⁺ sub-collector ~500 Å GaAs(Si-doped ~5 × 10¹⁸ cm⁻³) 8 N⁺ Buffer ~500 Å GaAs (Si-doped ~5 × 10¹⁸cm⁻³) 9 N⁺ GaAs conducting substrate Crystalline

Exemplary Configuration 3: an NPN and PNP GeSiSn Emitter-GeSnBase-GeSiSn Collector symmetric double heterojunction transistor. Thisdevice configuration is different because GeSiSn can be latticed matchto GeSn, even though the GeSiSn has a larger band gap energy than GeSn.In addition, because the ternary alloy GeSiSn can be grown at variouscompositions, it is possible to also biaxial tensile strain orcompressive strain the GeSn base region (lattice constants above andbelow GeSn). For GeSiSn the Sn % and Si % can be adjusted so that thelattice parameter remains constant. Also P-type and N-type doping havebeen achieved in GeSiSn. GeSiSn can be grown on Si, GaAs, Ge substrates.For exemplary configuration 3, Si substrates will be a possible choice.

For Si based HBTs, GeSiSn is a unique semiconductor alloy because it canbe latticed matched to Ge at the compositionGe_(1-x)(Si_(0.8)Sn_(0.2))_(x), where “x” can vary from 0 to 0.5 and thedirect gap energy of this material can vary from 0.8 eV to 1.24 eV. ThusGeSiSn is an excellent emitter for a Si HBT or a barrier layer for aGeSn quantum well or quantum dot, because it can be latticed matched toGeSn or can compressively strain the GeSn thus promoting island growthnecessary for quantum dot formation. By lowering the Si to Sn ratio inGeSiSn the lattice constant can be decreased.

FIG. 49 shows a possible exemplary flat band energy band diagram for asymmetric double HBT GeSiSn emitter-GeSn base-GeSiSn collector structure4900 which can work as an NPN or PNP transistor device. However, becauseGe_(1-x)(Si_(0.8)Sn_(0.2))_(x) 4901 & 4903 can be grown at differentalloy (x) compositions, both compressive and tensile strain can beapplied to the GeSn 4902 base, both configurations PNP and NPN areuseful. This figure shows the flat “Γ” band edge energy diagram of thematerial structure.

Additionally, the base can be graded from Ge to GeSn to have electricfield enhancement of the charge carriers (electrons and holes) as shownbelow in the “Γ” band edge diagram. FIG. 50 shows a possible exemplaryflat band energy band diagram for GeSiSn emitter-graded Ge—GeSnbase-GeSiSn collector structure double HBT 5000 which can work as an NPNor PNP transistor device. With the graded Ge—GeSn 5001 base region afield enhancement region 5002 is created in the base. Such structurecreates an electric field that accelerates the electrons and holesacross the base to the collector. The graded Ge—GeSn 5001 layer maycomprise at the emitter a Ge or a low Sn % GeSn layer which is graded tohigher Sn % GeSn at the collector interface. The grading range can gofrom Ge at the emitter to GeSn at various compositions up to 20%.

Table 8 shows a possible exemplary structure for a symmetric doubleheterojunction GeSiSn emitter-GeSn base-GeSiSn collector structure whichcan work as an NPN device. If the GeSiSn lattice constant is made largerthan the GeSn lattice constant, the GeSn can be tensile strained(similar to the InGaP emitter and collector situation). This causes thelight hole band to rise above the heavy hold band in the valence bandand results in a significant enhancement in the P-type GeSn basemobility and, thus, the same base thickness the base sheet resistancecan be reduced and the high frequency performance of the transistor is(F_(max)) increased. Additionally, because the hole mobility isenhanced, the base resistivity will be reduced, thus one has theadditional option to reduce the thickness of the base while keeping thebase sheet resistance unchanged. A thinner base promotes F_(T) toincrease. In this exemplary structure the base could be a P⁺ Ge layer.

TABLE 8 Epitaxial structure of an NPN heterojunction GeSiSn emitter -GeSn base - GeSiSn collector Layer Layer Name Description Comment 1 N⁺Emitter Cap ~1500 Å SiGe (As-doped ~5 × 10¹⁸ cm⁻³) 2 N⁻ Emitter ~500 ÅGe_(1−x)(Si_(0.8)Sn_(0.2))_(x) (As-doped ~3 × 10¹⁷ cm⁻³) 3 P⁺ Base ~500Å GeSn (B-doped >10¹⁹ cm⁻³) Or graded 0 ≦ Sn % ≦ 20% Ge—GeSn Thicknessrange 100 Å-5000 Å 4 N⁻ Collector ~10000 ÅGe_(1−x)(Si_(0.8)Sn_(0.2))_(x) (As-doped ~1 × 10¹⁶ cm⁻³) 5 N⁺sub-collector ~5000 Å SiGe (As-doped 5 × 10¹⁸ cm⁻³) SiGe Ge content canbe varied to accomodate the Ge_(1−x)(Si_(0.8)Sn_(0.2))_(x) layer 6 N⁺Buffer ~500 Å Si (As-doped 2 × 10¹⁸ cm⁻³) 7 N⁺ Si substrate Crystalline

Table 9 shows a possible exemplary structure for a symmetric doubleheterojunction GeSiSn emitter-GeSn base-GeSiSn collector structure whichcan work as a PNP device. If the GeSiSn lattice constant is made largerthan the GeSn lattice constant then the GeSn can be tensile strained.This cause the light hole band in the valence to split from the heavyhole band and results in an enhancement in the P-type GeSn base mobilitycan occur and thus reducing the base sheet resistance and increasing thehigh frequency performance of the transistor. In this exemplarystructure the base could be a heavily N⁺ Ge layer.

TABLE 9 Epitaxial structure of a PNP heterojunction GeSiSn emitter -GeSn base - GeSiSn collector Layer Layer Name Description Comment 1 P⁺Emitter Cap ~1500 Å SiGe (B-doped ~5 × 10¹⁸ cm⁻³) 2 P⁻ Emitter ~500 ÅGe_(1−x)(Si_(0.8)Sn_(0.2))_(x) (B-doped ~3 × 10¹⁷ cm⁻³) 3 N⁺ Base ~500 ÅGeSn (As-doped >10¹⁹ cm⁻³) Or graded Ge—GeSn 0 ≦ Sn % ≦ 20% Thicknessrange 100 Å-5000 Å 4 P⁻ Collector ~10000 ÅGe_(1−x)(Si_(0.8)Sn_(0.2))_(x) (B-doped ~1 × 10¹⁶ cm⁻³) 5 P⁺sub-collector ~5000 Å SiGe (B-doped ~5 × 10¹⁸ cm⁻³) SiGe Ge content canbe varied to accomodate the Ge_(1−x)(Si_(0.8)Sn_(0.2))_(x) layer 6 P⁺Buffer ~500 Å Si (B-doped ~2 × 10¹⁸ cm⁻³) 7 P⁺ Si substrate Crystalline

FIG. 51 shows a possible exemplary flat band energy band diagram for asymmetric double heterojunction transistor where a GeSn QW or QD isembedded in the GeSiSn base region with SiGe emitter/cladding and SiGecollector/cladding. This structure can work as an NPN or PNP transistorlight emitter device. Here a GeSn QW or QD 5103 is inserted into aGe_(1-x)(Si_(0.8)Sn_(0.2))_(x) 5102 & 5104 base region. The SiGe 5101emitter/cladding and the SiGe 5105 collector/cladding, form the majorindex difference for light confinement in the waveguide 5106 region, andalso have a large bandgap energy for to funnel carriers into the activeregion.

Table 10 shows a possible exemplary structure for a symmetric NPN doubleheterojunction transistor laser structure with an N⁻ SiGeemitter/cladding, a GeSn QD embedded in P⁺ GeSiSn base with an N⁻ SiGecollector/cladding.

TABLE 10 A symmetric NPN double heterojunction transistor laserstructure with a SiGe emitter/cladding, a GeSn QD embedded in GeSiSnbase with a SiGe collector/cladding. Layer Layer Name DescriptionComment 1 N⁺ cap ~1000 Å SiGe (As-doped >10¹⁹ cm⁻³) 2 N⁻ EmitterCladding ~4000 Å SiGe (As-doped ~5 × 10¹⁷) 3 P⁺ Base ~500 ÅGe_(1−x)(Si_(0.8)Sn_(0.2))_(x) (B-doped >10¹⁹ cm⁻³) 4 QW or QD undoped~55 Å GeSn Light emission Sn content can be: 0 ≦ Sn % ≦ 20% 1000 nm-4000nm QW (Sn % ~7% to 12% GeSn) QW thickness range 10 Å-1000 Å QD (Sn %~12% to 20% GeSn) QD size range 10 Å-200 Å 5 P⁺ Base ~500 ÅGe_(1−x)(Si_(0.8)Sn_(0.2))_(x) (B-doped >10¹⁹ cm⁻³) 6 N⁻Collector/Cladding ~4000 Å SiGe (As-doped ~5 × 10¹⁷ cm⁻³) SiGe Gecontent can be varied to accomodate the Ge_(1−x)(Si_(0.8)Sn_(0.2))_(x)layer 7 N⁺ sub-collector ~500 Å Si (As-doped ~5 × 10¹⁸ cm⁻³) 8 N⁺ Siconducting substrate Crystalline

Table 11 shows a possible exemplary structure for a symmetric PNP doubleheterojunction transistor laser structure with a P⁻ SiGeemitter/cladding, GeSn QD embedded in N⁺ GeSiSn base with a P⁻ SiGecollector/cladding.

TABLE 11 A symmetric PNP double heterojunction transistor laser withSiGe emitter/cladding, GeSn QD embedded in GeSiSn base and SiGecollector/cladding. Layer Layer Name Description Comment 1 P⁺ cap ~1000Å SiGe (B-doped >10¹⁹ cm⁻³) 2 P⁻ upper Cladding ~4000 Å SiGe (B doped ~5× 10¹⁷⁾ 3 N⁺ Base ~500 Å Ge_(1−x)(Si_(0.8)Sn_(0.2))_(x) (As-doped>10¹⁹cm⁻³) 4 QW or QD undoped ~55 Å GeSn Light emission Sn content can be: 0≦ Sn % ≦ 20% 1000 nm-4000 nm QW (Sn % ~7% to 12% GeSn) QW thicknessrange 10 Å-1000 Å QD (Sn % ~12% to 20% GeSn) QD size range 10 Å-200 Å 5N⁺ Base ~500 Å Ge_(1−x)(Si_(0.8)Sn_(0.2))_(x) (As doped >10¹⁹ cm⁻³) 6 P⁻Collector/Cladding ~4000 Å SiGe (B-doped ~5 × 10¹⁷ cm⁻³) SiGe Ge contentcan be varied to accomodate the Ge_(1−x)(Si_(0.8)Sn_(0.2))_(x) layer 7P⁺ sub-collector ~500 Å Si (B-doped ~5 × 10¹⁸ cm⁻³) 8 P⁺ Si substrateCrystalline

It should be noted that there are many types of N-type and P-typedopants. For standard III-V semiconductors like GaAs, InP, InGaAs,InGaP, the N-type dopants are Si, Ge, Sn, Pb, S, Se, Te. The P-typedopants for standard III-V semiconductors are C, Zn, Be, Mg. Commondopants for group IV semiconductors like GeSn, Ge, Si, SiGe, GeSiSn forN-type dopants are P, As, Sb. The P-type dopants are B, Al, Ga.

Exemplary Configuration 4, Si Emitter-SiGe base with GeSn QD-SiCollector transistor laser. The introduction of a GeSn quantum well intoa standard SiGe HBT design allows for the novel development of a Siphotonic transistor laser. SiGe has a wide range of band gaps as shownby the figure below, which is a plot of the Si_(x)Ge_(1-x) energybandgap as a function of x or Si content. To fabricate a light emittingbipolar transistor the following flatband diagram is shown for an NPNdevice. Inserted into the SiGe base is a GeSn quantum well. The baseregion could also be a thin Ge highly doped material. The figure showsan exemplary flat band energy diagram of the material structure.

FIG. 52 shows an exemplary flat band diagram of a Si Emitter-SiGe basewith GeSn QD-Si Collector light emitting HBT 5200. This HBT laser isgrown on Si substrates, thus compatible with Si processing. Here a GeSnQD 5204 has barriers region of Si_(0.8)Ge_(0.2) 5203 & 5205 P⁺base/barrier. The Si_(0.8)Ge_(0.2) forms the P⁺ base and also acts as abarrier layer for quantum confine the electrons and holes in the GeSn QD5204. For a QD the growth of a large lattice constant material on asmaller lattice constant material results in strained layer epitaxyallowing the self-assembled three dimensional island growth. Typicalquantum dot diameters are in the range of 1-20 nm, but are dependent onthe wavelength of light that needs to be emitted. The GeSn 5204 insertedinto a base/barrier serves for the collection region for electrons andholes to recombine to generate light. The Si_(0.8)Ge_(0.2) 5203 & 5205also serve as the optical confinement layer and the waveguide material.The Si 5202 & 5206 serves as the N⁻ emitter/cladding and N⁻collector/cladding material for this structure. The cladding serves asfunneling carriers into the active/waveguide region and traps the emitlight in the waveguide 5207 structure.

FIG. 53 shows a possible cross-sectional device depiction of a Si basededge emitting transistor laser or light emitting structure. Thetransistor laser includes a GeSn QD 5304 inserted into aSi_(0.8)Ge_(0.2) 5303 & 5305 P⁺ base/barrier of the HBT. The laser canrequire a resonant cavity to get optical gain, and typically this can beformed from the front and back cleaved facets of the semiconductorcrystal wafer. The structure can be grown on N⁺ Si conducting substrate5308, which is the seed crystal to grow the full structure. An N⁺ Sisub-collector 5307 is grown on the substrate. A N⁻ Si collector/cladding5306 and the N⁻ Si emitter/cladding 5302 do dual functions of opticalconfinement of the light 5309 produced form the active region GeSn QD5304 and the controlling the flow of electrons and holes. The P⁺Si_(0.8)Ge_(0.2) Base 5305 & 5303 form the barrier material for the GeSnQD 5304, and also provide the waveguide material. The laser can requirea resonant cavity to get optical gain, and typically this can formedfrom the front cleaved facets 5311 and back cleaved facets 5310 of thesemiconductor crystalline structure.

Table 12: Shows an exemplary structure that could be grown. Note forthis HBT device the Si_(0.8)Ge_(0.2) base could be graded down to lowerSi content or be replaced with a Ge base material.

TABLE 12 Epitaxial structure of NPN light emitting SiGe—GeSn—SiGe HBTLayer Layer Name Description Comment 1 N⁺ cap ~2000 Å Si (As-doped >10¹⁹cm⁻³) As = Arsenic 2 N⁻ Emitter/Cladding ~5000 Å Si (As-doped ~5 × 10¹⁷cm⁻³) 3 P⁺ Base ~500 Å Si_(.8)Ge_(.2) (B-doped >10¹⁹ cm⁻³) SiGe Could begraded 4 QD undoped ~55 Å GeSn Light emission Sn content can be: 0 ≦ Sn% ≦ 20% 1000 nm-3000 nm QD size range 10 Å-200 Å 5 P⁺ Base ~500 ÅSi_(.8)Ge_(.2) (B-doped >10¹⁹ cm⁻³) SiGe could be graded 6 N⁻Collector/Cladding ~5000 Å Si (As-doped ~5 × 10¹⁷ cm⁻³) 7 N⁺sub-collector ~2000 Å Si (As-doped ~5 × 10¹⁸ cm⁻³) 8 N⁺ Si conductingsubstrate Crystalline

FIG. 54 shows a laser structure, an additional variation of FIG. 52,because using higher Ge content in the Si_(0.6)Ge_(0.4), it can bepossible to produce QWs or QDs in this structure. FIG. 54 shows the flatband energy diagram of this structure. The laser includes a GeSn QW orQD 5404 inserted into a Si_(0.6)Ge_(0.4) P⁺ barrier/base 5403 & 5405.This HBT laser is grown on Si substrates, thus compatible with Siprocessing. The Si_(0.6)Ge_(0.4) forms the P⁺ base and also acts as abarrier layer for quantum confine the electrons and holes in the GeSn QWor QD 35404. The GeSn QW or QD 5404 inserted into a base/barrier servesfor the collection region for electrons and holes to recombine togenerate light. The Si_(0.6)Ge_(0.4) 5403 & 5405 also serve as theoptical confinement layer and the waveguide material. TheSi_(0.8)Ge_(0.2) 5402 & 5406 serves as the emitter/cladding andcollector/cladding material for this structure. The cladding serves asfunneling carriers into the active/waveguide 5407 region and traps theemitted light in the waveguide structure.

Table 13: Shows an exemplary structure that could be grown whichincludes Si_(0.6)Ge_(0.4) P⁺ base.

TABLE 13 Epitaxial structure of NPN light emitting SiGe—GeSn—SiGe HBT(with Si_(.6)Ge_(.4)) Layer Layer Name Description Comment 1 N⁺ cap~2000 Å Si_(.8)Ge_(.2) (As-doped >10¹⁹ cm⁻³) As = Arsenic 2 N⁻Emitter/Cladding ~5000 Å Si_(.8)Ge_(.2) (As-doped ~5 × 10¹⁷ cm⁻³) 3 P⁺Base ~500 Å Si_(.6)Ge_(.4) (B-doped >10¹⁹ cm⁻³) SiGe could be graded 4QW or QD undoped ~55 Å GeSn Light emission Sn content can be: 0 ≦Sn % ≦20% 1000 nm-4000 nm QW (Sn % ~7% to 12% GeSn) QW thickness range 10Å-1000 Å QD (Sn % ~12% to 20% GeSn) QD size range 10 Å-200 Å 5 P⁺ Base~500 Å Si_(.6)Ge_(.4) (B-doped >10¹⁹ cm⁻³) SiGe could be graded 6 N⁻Collector/Cladding ~5000 Å Si_(.8)Ge_(.2) (As-doped ~5 × 10¹⁷ cm⁻³) 7 N⁺sub-collector ~2000 Å Si (As-doped ~5 × 10¹⁸ cm⁻³) 8 N⁺ Si conductingsubstrate Crystalline

The transistor laser has the integrated features of both the transistorand the laser. From such a structure it can be straightforward tosimplify the structure and grow a separate confinement heterostructure(SCH) laser.

FIG. 55 shows an exemplary flat band energy diagram of an SCH laserutilizing a GeSn QW or QD region 5503 located in UIDGe_(1-x)(Si_(0.8)Sn_(0.2))_(x) barrier/OCL layer 5502 & 5504 region withP⁺ SiGe 5501 cladding and N⁺ SiGe 5505 cladding layers. This structurerepresents a PN junction or diode with an unintentionally doped (UID)active region and optical confinement region between the P⁺ SiGe 5501and N⁺ SiGe 5505 cladding. The UID Ge_(1-x)(Si_(0.8)Sn_(0.2))_(x) 5502 &5504 forms the barrier material for the GeSn QW or QD 5503 activeregion. The combination of the barrier and active region forms thewaveguide 5506 of the laser. The P⁺ SiGe 5501 cladding region serves forinjection of the holes and for the optical confinement of the lightemitted from the active region. The N⁺ SiGe 5505 cladding region servesfor injection of the electrons and for the optical confinement of thelight emitted from the active region in the waveguide 5506. The UIDGe_(1-x)(Si_(0.8)Sn_(0.2))_(x) 5502 & 5504 could be replaced with a Gebarrier layer as an alternative version of the SCH laser structure. TheGe barrier to GeSn would have a type I heterojunction alignment.

Table 14 shows an exemplary epitaxial structure SCH injection diodelaser with a GeSn QW or QD region with GeSiSn barriers.

TABLE 14 Exemplary epitaxial structure SCH injection diode laser with aGeSn QW or QD region with GeSiSn barriers. Layer Layer Name DescriptionComment 1 P⁺ cap ~1000 Å SiGe (B-doped >10¹⁹ cm⁻³) 2 P⁺ Cladding ~5000 ÅSiGe (B-doped ~1 × 10¹⁸) 3 UID Barrier/OCL ~500 ÅGe_(1−x)(Si_(0.8)Sn_(0.2))_(x) Also could use a Ge barrier 4 QW undoped~55 Å GeSn Light emission Or QD Sn content can be: 0 ≦ Sn % ≦ 20% 1000nm-4000 nm QW (Sn % ~7% to 12% GeSn) QW thickness range 10 Å-1000 Å QD(Sn % ~12% to 20% GeSn) QD size range 10 Å-200 Å 5 UID Barrier/OCL ~500Å Ge_(1−x)(Si_(0.8)Sn_(0.2))_(x) Also could use a Ge barrier 6 N⁺Cladding ~5000 Å SiGe (As-doped ~1 × 10¹⁸ cm⁻³) SiGe: Ge content can bevaried to accomodate the Ge_(1−x)(Si_(0.8)Sn_(0.2))_(x) layer 7 N⁺Buffer ~5000 Å Si (As-doped ~5 × 10¹⁸ cm⁻³) 8 N⁺ Si conducting substrateCrystalline

Exemplary Configuration 5A: An NPN GaAs Emitter-GeSn Base-GaN Collectordouble heterojunction bipolar transistor with dissimilar materials. Thisdevice configuration comprises an emitter/base stack of GaAs—GeSn waferbonded to a GaN collector. GaN with its high band gap offers tremendousimprovements in the breakdown voltage of the HBT. The device elucidatedcan include a double heterojunction GaAs—GeSn—GaN HBT device. The adventof device technology based on GaN with its high electric field strengthis a new direction for high-power RF amplification. GaN based materialshave a large bandgap and high electron saturation velocity. Theembodiments described herein demonstrate a new semiconductor transistorIC with ultra-high performance in applications requiring both high speedand high power rugged electronics. In examples described herein, the GaNcan be grown on the various substrates like sapphire, SiC, Si GaAs, GaNand template substrates.

Polar GaN wurtzite structure can be grown on sapphire, SiC (manypolytypes: 3C, 4H, 6H, etc.), Si substrates, or template substrates andhas piezoelectric and polarization charge. GaN grown in the wurtzite(hexagonal) phase results in large spontaneous and piezoelectricpolarization charge.

Non-polar GaN cubic (FCC) structure can be grown on GaAs, Si, ortemplate substrates. GaN in this form has no polarization charge. Acubic form of GaN with (001) orientation can be grown on zinc blendGaAs. Thus the cubic GaN can be grown on conducting GaAs which can actas the sub-collector. The zinc-blend (cubic) GaN collector has anegligible conduction band offset with respect to the GeSn base. Theconduction band offset between GaAs and cubic GaN is roughlyΔE_(C)˜−0.024 eV. Thus the GeSn (close to Ge) the conduction band offsetto GaN is about ΔE_(C)˜0 eV at the base/collector heterojunction.Additionally, non-polar wurtzite forms can be cut from the c-planegrowth along the “a” or “m” plane directions. If the GaN is grown alongthe m or a-plane axis, these polarization effects can be eliminated.Typical GaN wurzite crystals grown along the direction (c-plane) ofIII-nitrides suffer from polarization induced electric fields. Electricfields do not exist across the along nonpolar directions a-plane orm-plane). Thus, high quality non-polar GaN substrate crystals areproduced by slicing a c-plane GaN boule along the “a” or “m” plane. Thisresults in low defect density non-polar substrates, which have improvedsubstrates for fabrication of devices.

The GaAs—GeSn—GaN heterojunction transistor described herein representsa revolutionary jump in both high power and high frequency performance.This device embodies enormous RF power output, ruggedness, highbandwidth, and good linearity, combined with low turn-on voltage, whichis desirable for minimizing power consumption. This unique arrangementof materials combines the high transconductance of heterojunctionbipolar transistor (HBT) technology, with the enormous breakdown voltage(using a GaN collector), and a desirable emitter-base heterojunction(wide band-gap GaAs emitter on a narrow band-gap high conductivityP-type GeSn base). The huge breakdown field of GaN allows the use ofshort collector devices with high bandwidths (e.g. cut-off frequencyF_(t) and maximum oscillation frequency F_(max)>than 150 Ghz). Thecombination of a low band-gap (<0.66 eV) GeSn base coupled with a wideband-gap GaN (˜3.4 eV) collector can be used for high speed powerapplications. By using a vertical stack of junctions, the device layersare shorter, resulting in lower resistances and shorter transit delays,both contributing to much higher frequencies. The use of efficientGaAs—GeSn—GaN transistors can significantly enhance battery life whilealso enabling operation at high powers with exceptional frequencyresponse. Ultra high performance transistors that can operate at highertemperatures, higher power densities, higher voltages and higherfrequencies are desirable for next-generation commercial applications(IT, consumer, automotive, industrial, etc.).

By utilizing various crystal growth technologies, pulse laser ablationepitaxy, molecular beam epitaxy, metal organic chemical vapordeposition, liquid phase epitaxy, vapor phase epitaxy, or various otherepitaxial growth techniques for the growth of base-emitter stack of P⁺GeSn base onto the N⁻ GaAs emitter thus forming the base-emitter stack,because at low Sn % lattice of GeSn is close to that of GaAs. Then theGeSn—GaAs emitter stack can be coupled with the exemplary wafer bondingtechnology can be merged to the GaN collector as described herein, thusforming a monolithic GaAs(emitter)-GeSn(base)-GaN(collector)semiconductor stack that is a desirable HBT embodiment for high-power,high-frequency electronics can be created. In some examples, theuniqueness of embodiments can result in a near zero conduction bandoffset through the three different semiconductor materials(GaAs—GeSn—GaN). New materials are required to build high powerelectronics that can also operate at frequencies in the 10 to 100 GHzrange. The formation of near lattice-matched GeSn on GaAs then waferbonded to GaN is a possible key to the realization of these devices.

FIG. 56 shows the energy band gaps of various semiconductors vs. theirlattice constant. The graph has a vertical axis with the values of thebandgap energy (eV) and a horizontal axis with the values of the Latticeconstant (Å). Various semiconductors are plotted as a function of theirbandgap energy and lattice constant. The condition of lattice matchingconstrains certain combinations of semiconductors. It is readily seenthat new types of semiconductor devices can be formed if the latticematching constraint were eliminated. It would then be possible tooptimize new types of heterojunctions based on the optimized materialscharacteristics instead of those constrained to near lattice constantmaterials.

The merging of the GeSn base region with the GaN collector by utilizingthe wafer bonding process for fabrication of heterogeneous materialsdescribed herein. With this approach, the GeSn and GaN epitaxial layerscan be joined to make a single composite structure. Monolithic waferbonding is an advanced process for forming PN junctions. This waferbonding technique allows formation of a robust monolithic structure,where the interface is covalently bonded. The new composite materialestablishes the GeSn—GaN base-collector heterointerface. Wafer bondingallows for the formation of a heterointerface without having to performheteroepitaxy of two poorly latticed matched materials.

The new HBT has a base-collector junction comprising the GeSn P⁺ baseregion wafer bonded to the GaN N⁻ collector is described herein. GeSnlattice constant can vary from 5.65 Å to 5.833 and lattice constant ofGaN is 4.4 Å, which is a huge mismatch (>28%). Such a mismatch does notallow for single or unstrained crystal structures, because the criticalthickness for the base to be grown on the collector would be thin. Withour approach, the GeSn and GaN epitaxial layers can be joined to make asingle composite crystalline structure. The wafer bonding techniquedescribed herein allows us to form a junction that is a robustmonolithic structure, where the interface is covalently bonded.

FIG. 57 shows the exemplary wafer bonding process that enablesmonolithic joining of two dissimilar semiconductor materials. Byemploying pressure, heat, gas ambient, and time, covalently bondedcomposite structures can be developed. The new composite materialestablishes the critical GeSn—GaN base-collector heterointerface. Waferbonding allows formation of a heterointerface without having to performheteroepitaxy of two poorly matched materials. Exemplary wafer bondingmethodology comprises (5701) the GeSn 5704 and GaN 5705 semiconductorsare cleaned in preparation for joining, (5702) the GeSn 5704 and GaN5705 are placed on each other in between the wafer bonder top plate 5706and wafer bonder bottom plated 5707, basically the jaws of the waferbonder, and held under heat 5709 and pressure 5708 for the requisitetime and in a gas ambient, then (5703) the final structure is amonolithic composite material with GeSn 5704 bonded to GaN 5705. TheGaAs—GeSn—GaN material structure avoids the use of ternary alloysemiconductors thereby making the epitaxial process less complex andeliminating alloy scattering of electrons.

NPN GaAs—GeSn—GaN HBTs can include the following concepts: Growth ofnear lattice matched P-type GeSn base on N-type GaAs emitter (GeSn/GaAsstack) because the lattice constant of Ge is almost the same as GaAsthus for low content Sn, GeSn has a slightly larger lattice constantthan GaAs. Monolithic formation by wafer bonding of GeSn/GaAs stack tothe N-type GaN (to circumvent large lattice mismatched growth). One ofthe advantages of the embodiments described herein is the formation of aunique transistor semiconductor stack that can have a near zeroconduction band offset between all three materials, with each materialoptimized for overall HBT performance.

FIG. 58 shows an exemplary flat band energy diagram wafer bonded NPNGaAs—GeSn—GaN HBT 5800. Here an emitter up emitter-base stack 5805comprising of N⁻ emitter GaAs 5802 grown on P⁺ Base GeSn 5803 structure.The full monolithic structure can be formed using an epitaxial liftoff(ELO) procedure. This emitter-base stack 5805 is wafer bonded to the N⁻collector GaN 5804 thus forming a wafer bonded junction 5801 at thebase-collector interface. The methodology of wafer bonding thisemitter-base stack to the GaN is described in Exemplary ELO WaferBonding Configuration 6A. Note the conduction band offset ΔE_(C) isapproximately near zero through the NPN HBT structure.

Additionally, the base can be graded from Ge—GeSn to have electric fieldenhancement of the charge carriers. FIG. 59 shows an exemplary flat bandenergy diagram of the NPN GaAs, graded Ge—GeSn, GaN HBT 5900. Here abase up emitter-base stack 5905 comprises P⁺ Base compositionally gradedGe—GeSn 5903 and grown on an N⁻ emitter GaAs 5902 structure. Thecompositionally graded Ge—GeSn 5903 layer may comprise at the emitterinterface a Ge or a low Sn % GeSn layer which is graded to higher Sn %GeSn at the collector interface. The compositional grading range can gofrom Ge at the emitter to GeSn at various compositions up to 20%. Thisemitter-base stack 5905 is then wafer bonded with the P⁺ Base next tothe N⁻ collector GaN 5804 thus forming a wafer bonded junction 5901 atthe base-collector interface. Due to the compositional grading of theGe—GeSn 5903 in the P⁺ Base, there is a field enhancement region 5906that accelerates the carriers toward the collector. The methodology ofwafer bonding this emitter-base stack is described in Exemplary InvertedWafer Bonding Configuration 6B. Note the conduction band offset ΔE_(C)is small through the NPN HBT structure.

FIG. 60 shows an exemplary cross-sectional device depiction of the waferbonded GaAs—GeSn—GaN NPN double HBT 6000 in a mesa configuration. Notethat this is a vertical device, which is desirable for powerapplications because the lateral area can be minimized. The device isgrown on an N⁺ SiC 4H substrate 6006. The NPN HBT comprises an emitterbase stack 6001 with a wafer bond 6010 to the GaN structure 6002 to formthe monolithic device. The GaN structure 6002 can comprise a variety offorms, but for an exemplary case the GaN is grown on a SiC substrate,though a GaN, sapphire, GaAs, Si substrate could also be used. Startingwith an N⁺ SiC substrate 6006, which can be of the 3C, 4H, 6H, etc.variety, on which an N⁺ SiC buffer 6007 is grown. Then an N⁺ SiCsub-collector 6008 can be grown, onto which an N⁻ GaN collector 6009 isgrown. This finalizes the GaN structure 6002. Because the GaN is grownon 4H SiC (wurtzite or hexagonal) this results in a polar GaN collector6009. For the emitter-base stack 6001 an ELO procedure to be describedin Configuration 6A, forms the N⁺ GaAs contact 6003—the N⁻ GaAs emitter6004 and the P⁺ GeSn base 6005, finalizing the emitter base stack 6001.P⁺ GeSn base 6005 layer forms the heavily doped P-type base. GeSn alsohas a large hole mobility which is a precondition for making the baseregion thin. The conduction band offset between GaAs—GeSn is almostzero, thus the majority of the 0.75 eV bandgap difference appears in thevalence band. The GaN can serve as the collector layer with a largebreakdown voltage for the transistor because it has a large bandgapenergy of 3.2 eV.

Table 15 is an exemplary Epitaxial structure of an NPN GaAs—GeSn—GaNwafer bonded HBT. In this structure, the GaN is grown on SiC which is atypical substrate for the growth and one of the many describedsubstrates that can be used. SiC is preferred for high power electronicsbecause it has the highest thermal conductivity between GaN, Si, GaAsand sapphire the other primary substrates for GaN.

TABLE 15 Epitaxial structure of an NPN GaAs—GeSn—GaN wafer bonded HBTLayer Layer Name Description Comment 1 N⁺ cap ~1000 Å InGaAs (Te-doped>10¹⁹ cm⁻³) 2 N⁻ Emitter Cap ~1500 Å GaAs (Si-doped ~5 × 10¹⁸ cm⁻³) 3 N⁻Emitter ~500 Å GaAs (Si-doped ~3 × 10¹⁷ cm⁻³) 4 P⁺ Base ~500 Å GeSn(B-doped >10¹⁹ cm⁻³) Or graded Ge—GeSn 0 ≦ Sn % ≦ 20% Thickness range100 Å-5000 Å 5 N⁻ Collector ~10000 Å GaN (N-doped ~1 × 10¹⁶ cm⁻³) WaferBonded to above 6 N⁺ sub-collector ~5000 Å SiC (N-doped ~5 × 10¹⁹ cm⁻³)7 High Purity Buffer ~500 Å SiC (N-doped ~5 × 10¹⁸ cm⁻³) N = nitrogen 8N⁺ SiC (4H) conducting substrate Crystalline

FIG. 61 shows another possible exemplary cross-section device depictionof the wafer bonded GaAs—GeSn—GaN NPN double HBT 6100 in a mesaconfiguration. Note that this is a vertical device, which is desirablefor power applications because the lateral area can be minimized. Inthis case the device is grown on a face centered cubic (FCC) N⁺ GaAssubstrate 6106. The NPN HBT comprises an emitter base stack 6001 with awafer bond 6110 to the GaN structure 6102 to form the monolithic device.The GaN structure 6102 can comprise a variety of forms, but for anexemplary case the GaN is grown on a GaAs substrate, though a GaN,sapphire, SiC, Si substrate could also be used. Starting with an N⁺ GaAssubstrate 6106, which a N⁺ GaAs buffer 6107 is grown. Then an N⁺ GaAssub-collector 6108 can be added, onto which an N⁻ GaN collector 6109 isgrown. This finalizes the GaN structure 6102. Because the GaN is grownon FCC GaAs (cubic) this results in a non-polar GaN collector 6109. AnELO procedure in Configuration 6A, describes how the emitter-base stack6001 is wafer bonded to the GaN collector structure. P⁺ GeSn base 6005layer forms the heavily doped P-type base. GeSn also has a large holemobility which is a precondition for making the base region thin. Theconduction band offset between GaAs—GeSn is almost zero, thus themajority of the 0.75 eV bandgap difference appears in the valence band.The GaN can serve as the collector layer with a large breakdown voltagefor the transistor because it has a large bandgap energy of 3.2 eV.

In some examples, formation of this monolithic composite material of anNPN GaAs—GeSn—GaN wafer bonded HBT can create a desirable devicearchitecture in that the conduction band offsets are near zero for bothemitter-base and base-collector hetero-interfaces and the valence bandoffset is large at the emitter-base GaAs—GeSn and base-collectorGeSn—GaN heterojunctions. This property allows for the formation ofheterojunction transistor structure that can have large gain (largevalence band offset between GaAs and GeSn), low base sheet resistance(GeSn has high hole mobility) and, low turn-on voltage (GeSn has lowband-gap energy), and large breakdown voltage (GaN has large breakdownelectric field strength and high saturated velocity), which aredesirable device metrics for next-generation electronic transistors. TheGaAs—GeSn—GaN HBT has the gain of GaAs, the huge breakdown voltage forrobustness, the high frequency performance greater than GaAs HBTs, thelow turn-on voltage of GeSn, improved electron transport because of thenear zero conduction band offset between the emitter-base-collector.Electrons can easily be injected from the GaAs emitter through the GeSnbase to the GaN collector. By adding the ability to grade the basecomposition from Ge—GeSn such that the bandgap energy of the material isgradually reduced throughout the base as described in ourconfigurations. This grading causes an electric field, which in turnreduces the transit time, thus increasing F_(t). The GaAs—GeSn—GaNmaterials stack can be desirable for making NPN HBTs that can outperformstandard SiGe, GaAs, and InP heterojunction bipolar transistors.

Exemplary GaAs emitter advantages: The large valence band offset betweenGaAs emitter and GeSn base can stop back injection of holes into theemitter. This is desirable because the base is doped heavily P-type(typically>1×10¹⁹ cm⁻³), with such high doping of the base, theemitter-base valence band offset blocks the holes even though the basedoping is much higher than the N-type emitter doping (low 10¹⁷ cm⁻³).This allows for low N-type doping of the emitter and high P-type dopingof the base, thus lowering base emitter capacitance while stillachieving sizable current gain. GeSn is near latticed matched to GaÅs(5.65 Å), which enables dislocation free growth. GaAs—GeSn—GaN materialstructure avoids the use of ternary alloy semiconductors thereby makingthe epitaxial process less complex and eliminating alloy scattering ofelectrons. The use of a GaAs (instead of InGaP) emitter and GaN (insteadof GaAs) collector significantly increases the overall thermalconductivity of the material structure. The GaAs—GeSn emitter basejunction has a large valence (at least 0.72 eV which is larger than theGeSn band gap). This eliminates the back injection of holes to theemitter from the base, which reduces the gain of the transistor.

Exemplary GeSn Base Advantages (low Sn % similar properties to Ge): GeSnhas a low band gap (lower than Ge: the term low energy band gap basetypically refers to the relevant semiconductors with band gaps less than0.75 eV, like GeSn, Ge, InGaAs, GaAsSb) which results in low turn-onvoltage (less than 0.5 V). GeSn (low Sn %) hole mobility is high (2000cm²/Vs) like Ge and acceptors can be incorporated to high density(>1×10¹⁹ cm⁻³), thus the base can be made ultra-thin while maintaining alow base sheet resistance which increases current gain and decreaseselectron transit time. By adding the ability to grade the basecomposition from Ge to GeSn such that the bandgap energy of the materialis gradually reduced throughout the base as described in ourconfigurations. This grading causes an electric field, which in turnreduces the transit time, thus increasing F_(t). GeSn can be made tobecome a direct gap semiconductor (unlike Ge which is an indirectsemiconductor). GeSn for low Sn concentration has shallow acceptors, sothe hole concentration is generally equal to the acceptor doping leveland independent of temperature. GeSn can be heavily doped P-type. Thelow base sheet resistance (<<less than 18 Ohm/sq) results in a highF_(max). (>than 150 GHz), GeSn hole resistivity should be on the orderof 0.0002 Ohm-cm. The surface recombination velocity is low for P-typeGe and GeSn.

Exemplary GaN Collector advantages: GaN collector can be grown on GaN,SiC (many polytypes, i.e. 3C, 4H, 6H), Si, Sapphire, GaAs and templatesubstrates. Thus the substrate can be chosen to optimize the propertiesof the device. For instance for high power devices it can be useful togrow the GaN on SiC substrates because they have a thermal conductivity.Typically GaN comes in the wurtzite and cubic phase. GaN (wurzite andcubic form) has a large lattice mismatch with GeSn, thus wafer bondingcircumvents the problem of growing strained and incompatible layers. GaNhas high breakdown field, which is excellent for the collector breakdownvoltage. GaN has near zero conduction band offset with GaAs, thus noblocking field at the interface. The use of GaN (instead of GaAs)collector significantly increases the overall thermal conductivity ofthe material structure. GaN has a high saturation velocity, thuselectrons travel without intervalley scattering.

Furthermore, because GeSn has a low resistivity of 0.0002 ohm-cm, onecan decrease the thickness of the base significantly, while stillmoderately increasing the base sheet resistance value. The frequencyresponse of the device is related to the F_(t) and F_(max). Therelationship between transit frequency F_(t) and the maximum oscillationfrequency F_(max) is as follows for an HBT:F_(max)=(F_(t)/8uR_(B)C_(CB))¹¹². The transit frequency F_(t) isbasically the inverse of the time for the electron to traverse theemitter, base and collector. The parameters R_(B) and C_(CB) refer tothe base sheet resistance and the capacitance of the collector basejunction. The parameter F_(max) is the unity power gain frequency andindicates the maximum frequency with power gain from a device. Thetransit frequency can be further improved by having a higher saturationvelocity which the GaN collector exhibits.

The NPN GaAs—GeSn—GaN HBT (referred to as GeSn HBT) as compared tostandard NPN InGaP—GaAs—GaAs HBT (referred as GaAs HBT) would have thefollowing mainly advantages. The differences between the two devices aretypically independent of the emitter, and mostly rely on the basematerial GeSn and the collector material GaN.

Advantage 1: If the GaAs HBT had a base thickness of 1000 Å, for theGeSn HBT base thickness could be halved to 500 Å, The F_(t) for GeSn HBTwould increase because of the thinner base. Because the GeSn baseresistivity (0.0002 ohm-cm) is 10 times less than GaAs resistivity(0.002) ohm-cm), the parameter F_(max) would increase by a factor of(5*F_(t))^(1/2). This is because F_(t) increased because the basethickness was halved, but the base sheet resistance of the GeSn baseonly increased by a factor of two, but it is still 5 times less than thebase sheet resistance of the GaAs HBT. Furthermore, by using GaN(1.5×10⁵ m/s) which has a higher saturation velocity than GaAs (1×10⁵m/s), this results in faster electron transit time across the GaNcollector.

A commonly used metric used for comparing various semiconductors is theJohnson's figure of merit (FOM), which can compares differentsemiconductors for suitability for high frequency power transistorapplications. Table 16 shows a comparison Johnson FOM for Si, GaAs, andGaN.

TABLE 16 Johnson FOM for Si, GaAs, GaN Si GaAs GaN Energy Bandgap (eV)E_(gap) 1.1 1.4 3.4 Electron Saturation Velocity (cm/s) V_(sat) 1 × 10⁷  1 × 10⁷ 1.5 × 10⁷ Peak Electron Saturation Velocity 1 × 10⁷ 1.9 × 10⁷2.5 × 10⁷ (cm/s) V_(sat peak) Normalized Johnson Figure of Merit 1   9.5572    (E_(gap) ⁴ × V_(sat peak) ²) Johnson Figure of Merit = Maximumpower times frequency ≈ E_(gap) ⁴ × V_(sat) ²

Advantage 2: Because GaN has such large breakdown voltage for examplefor a Ge—GaN base collector junction where the GaN collector is only5000 Å thick, the breakdown voltage is greater than 100 V. A typicalGaAs heterojunction base-collector, where the GaAs collector is over10000 Å thick has a breakdown voltage of only 20 volts. Thus one canreduce the GaN collector thickness as compared to the GaAs, withouthurting robustness, thus the transit time or F_(t) is increased becauseof a thinner base and collector, which in turn increases the F_(max)with an additional factor because of the low resistivity GeSn base.Typical values of F_(t) and F_(max) should be greater than 150 GHz.

Exemplary Wafer Bonding of GeSn Stack to GaN: The method of waferbonding is chosen as the most direct means of forming the GeSn to theGaN structure. The bonder, shown and described below, can eliminate theproblems associated with the more conventional torqued jig fixtures.Using the method described here, the bonder allows gradual pressureapplication for the delicate bonding of GeSn and GaN. The large sizeheaters in the plates provide fast temperature ramp up for the bondingprocess. The bonder has a self-leveling action to the surface mechanismand ensures that it is flat with the surface. We have developeddifferent wafer bonders and as well as different wafer bondingprocesses. QuantTera's custom wafer bonders have two independenttemperature controllers to precisely control the temperature of the topand bottom bonding plates.

Our wafer bonders have a unique feature that the top and bottom platesare under electronically controlled differential air pressure. There isno non-linear return spring force needing to be concerned. The top platemoving up and down relies on the differential air pressure in the topplate's air cylinder; and thus, the bonding pressure can be continuouslyadjusted precisely to provide optimized wafer bonding conditions. Ourwafer bonding system has precise temperature and pressure control toensure the bonding of the materials. Operation step 1 comprises loweringthe wafer bonder top plate so that it barely touches the materials to bebonded. Pressure is then slowly applied at this time and the temperatureof the bonder top and bottom plate are raised. Independent temperaturecontrol of the top and bottom plate temperatures allows theaccommodation of materials that may have different thermal expansioncoefficients, thus minimizing stress to the bonded interface. Ourbonders can reach temperatures above 500° C. in various gas ambients,but typically a nitrogen purge is used during the bonding process. Ourbonders can accommodate up to 4″ diameter wafers. In a single step wecan easily achieve a PN homojunction or heterojunction bonded materials.

FIG. 62 shows QuantTera's wafer bonder configuration 6200. The bonderuses differential air pressure between P1 pressure 6201 and P2 pressure6202, where the pressure is measured by the differential pressure gauge6203. The pressure controls the action of the top plate 6204 in movingdown to clamp the device and substrate 6207, which sits on the bottomplate 6205, which has a ball bearing 6206 for conformal leveling action.Two independent temperature controllers control the temperature of thetop plate 6204 and bottom plate 6205.

Table 17 shows an exemplary wafer bonding process.

TABLE 17 Exemplary wafer bonding process Step Description 1 The wafer iscleaved to appropriate size. 2 Semiconductor materials are thoroughlycleaned. 3 Oxides are removed from surface by chemical etch or plasmaetcher. 4 GeSn stack material and the GaN material are placed on top ofeach other. 5 GeSn and GaN materials are placed in wafer bonder, whichunder pressure joins the materials together at typical temperatures of300-600° C. for 1 to 2 hours. In a gas ambient. 6 The compositestructure is cooled and then removed. 7 The composite unit acts as amonolithic PN structure. 8 Current voltage testing of the PN junction 9Shear test to see the strength of wafer bonded junction

Wafer bonding allows formation of a heterointerface without having toperform heteroepitaxy of two poorly matched materials. FIG. 63 shows thecurrent-voltage characteristic of the wafer bonded P GeSn to N—GaNshowing PN rectifying behavior. The vertical axis is current in units ofmA, and the horizontal axis is voltage in units of V. The turn-onvoltage of the device is less than 0.5 V.

The wafer bonding process allows for independent optimization ofmaterials without regard to lattice matching. It should be noted that Gelattice constant is about 5.65 Å and GaN is 4.4 Å, which is a hugemismatch (28%). Interface defects can be minimized by varying waferbonding parameters such, as oxide removal, temperature, time, andpressure. Table 18 lists the thermal expansion coefficients of the GaAs,GeSn, and GaN. Because the thermal expansion coefficients of all thematerials are similar, the thermal stress generated during wafer bondingshould be minimal.

TABLE 18 Thermal Expansion Coefficients of GaAs, GeSn, GaN MaterialThermal Expansion Coefficient (10⁻⁶ K⁻¹) @ 300 K GaAs 6.0 GeSn similarto Ge 5.9 GaN 5.6

Possible advantages of the GaAs—GeSn—GaN HBT devices described herein:The GaAs—GeSn—GaN stack minimizes the conduction band offsets, whichhinder electron transport (ultra-fast transistor action). Thesemiconductor materials are optimized for performance: GaN collector;GeSn base; and GaAs emitter. GaAs and GeSn are near latticed matched andwafer bonding allows for the integration of GaN without having toperform lattice-mismatch growth (Ge is latticed matched to GaAs, thusGeSn for low content Sn has a slightly larger lattice constant thanGaAs). Wafer bonding is desirable for GaAs—GeSn—GaN because the thermalexpansion coefficients are close to each other. The GaAs—GeSn—GaN HBTexceeds SiGe, InP and GaAs HBTs in terms of turn-on voltage, has muchlower base sheet resistance allowing for a much thinner base, and thebreakdown field that is 10 times higher. Two fundamental obstacles toconventional GaN HBTs are the high resistivity and large bandgap energyof the base layer. This results in a HBT with high base sheetresistivity and large turn-on voltage. Most of the GaN NPN HBTs haveutilized complex re-growth strategies in an attempt to address theseproblems. Despite limited success with regard to DC transistorproperties, these issues remain as impediments to high frequencyoperation of conventional GaN HBTs. The GaAs—GeSn—GaN HBT describedherein solves all these issues and outperform the technology oftraditional systems. Prominent commercial markets exist where theGaAs—GeSn—GaN transistor described herein can be implemented: 1)cellular handset market, 2) RF high power electronics.

Exemplary Configuration 5B: NPN InGaP Emitter-GeSn Base-GaN CollectorDouble HBT 6200 with all dissimilar materials Desirable combination ofsemiconductors (near Zero Conduction Band Offset betweenEmitter-Base-Collector). To further improve on configuration 5A, anInGaP emitter region is added that is lattice matched to GaAs. Thisdevice comprises an emitter stack of InGaP—GeSn wafer bonded to a GaNcollector. GaN with it high band gap offers tremendous improvements inthe breakdown voltage of the HBT. Note the InGaP layer can becompositionally graded to enhanced device performance. The monolithicInGaP—GeSn—GaN stack is unusual in that the conduction band offset isnear zero. This special property allows for the formation ofheterojunction transistor structure that can have large gain, and largebreakdown voltage (GaN has large breakdown electric field strength andhigh saturated velocity). These material characteristics can make adesirable bipolar transistor. The InGaP—GeSn—GaN materials stack can bedesirable for making NPN HBTs that can outperform standard SiGe, GaAs,and InP heterojunction bipolar transistors.

FIG. 64 shows the exemplary flat band energy band diagram of NPN InGaPEmitter-GeSn Base-GaN Collector Double HBT 6400. This new materialstructure with near zero conduction band offsets between interfaces anda large valence band offset at the emitter-base and base collectorheterojunction. Electrons can easily be injected from the InGaP emitterthrough the GeSn base to the GaN collector. Conduction band offsets atemitter-base and base-collector junctions are near zero, with largevalence band offsets between the InGaP—GaAs and GaAs—GaNheterojunctions. The band alignments are desirable for high performanceHBTs. Here an emitter up emitter-base stack 6405 comprising N⁻ emitterordered InGaP 6402 grown on P⁺ Base GeSn 5803 structure. The orderedInGaP 6402 should have the smallest conduction band offset ΔE_(C) withthe GeSn 5803. This emitter-base stack 6405 is then wafer bonded to theN⁻ collector GaN 5804 thus forming a wafer bonded junction 6401 at thebase-collector interface. The methodology of wafer bonding thisemitter-base stack is described in Exemplary ELO Wafer BondingConfiguration 6A. Note the conduction band offset ΔE_(C) isapproximately near zero through the NPN HBT structure.

InGaP semiconductor can be grown epitaxially and latticed matched toGaAs at the composition In_(0.49)Ga_(0.51)P. If typically grown at hightemperatures, it can grow in an ordered phase where the crystallinestructure forms sheets of In—P and Ga—P atoms can alternate in the (001)planes of the FCC unit cell without the intermixing of the Ga and Inatoms on the lattice planes. This results in an almost zero conductionband discontinuity between the InGaP and GaAs and is called the orderedphase (this can be of weakly type I or weakly type II because it isclose to zero). With different growth conditions, the In and Ga atomscan intermix and the disordered InGaP phase can form, which has aconduction band offset (0.1 eV vs. 0.03 eV for the ordered phase). Ineither case the conduction band offset of InGaP to GaAs or GeSn issmall.

Additionally, the base can be graded from Ge—GeSn to have electric fieldenhancement of the charge carriers. FIG. 65 shows an exemplary flat bandenergy diagram of the NPN InGaP-graded Ge to GeSn—GaN HBT 6500. Here forvariation a base up emitter-base stack 6505 comprising P⁺ Basecompositionally graded Ge—GeSn 6503 grown on a N⁻ emitter disorderedInGaP 6502 structure. The compositionally graded Ge—GeSn 6503 layer maycomprise at the emitter a Ge or a low Sn % GeSn layer which is graded tohigher Sn % GeSn at the collector interface. The compositional gradingrange can go from Ge at the emitter to GeSn at various compositions upto 20%. This emitter-base stack 6505 is then wafer bonded with the P⁺Base next to the N⁻ collector GaN 5804 thus forming a wafer bondedjunction 6501 at the base-collector interface. Due to the compositionalgrading of the Ge—GeSn 6503 in the P⁺ Base, there is a field enhancementregion 6506 that accelerates the carriers toward the collector. Themethodology of wafer bonding the emitter-base stack to the GaN isdescribed in Exemplary Inverted Wafer Bonding Configuration 6B. Note theconduction band offset ΔE_(C) is small through the NPN HBT structure.Note the differences between flat band energy diagrams FIG. 63 with theordered InGaP and FIG. 64 with disordered InGaP is small.

Exemplary InGaP (In_(0.49)Ga_(0.51)P) Emitter Advantages. The largevalence band offset between InGaP emitter and GaAs base stops backinjection of holes into the emitter. This allows for low N-type dopingof the emitter and high P-type doping of the base, thus lowering baseemitter capacitance while still achieving sizable current gain. The nearzero band conduction offset between the InGaP and the GeSn base can bedesirable for electron injection into the base layer. Ordered InGaP mayhave reduced temperature sensitivity to the current gain. InGaP can belatticed matched to GaAs or Ge or GeSn (low Sn %), which enablesdislocation free growth.

Table 19 shows an exemplary epitaxial structure of NPN InGaP—GeSn—GaNHBT grown and wafer bonded. In this structure the GaN is wurtzitehexagonal structure, a or m plane material. GaN in this form has nopolarization charge that degrades the base-collector performance.Non-polar GaN wurzite substrates are illustrated here though one coulduse SiC, GaAs, Si, sapphire (non-polar and polar forms). GaN (FCC) canalso be grown on GaAs which also lacks polarization charge effects.

TABLE 19 Epitaxial Structure of NPN InGaP—GeSn—GaN HBT Layer Layer NameDescription Comment 1 N⁺ cap ~1000 Å InGaAs (Te-doped >10¹⁹ cm⁻³) 2 N−Emitter Cap ~1500 Å GaAs (Si-doped ~5 × 10¹⁸ cm⁻³) 3 N− Emitter ~500 ÅInGaP (Si-doped ~3 × 10¹⁷ cm⁻³) Ordered or disordered 4 P⁺ Base ~500 ÅGeSn (B-doped >10¹⁹ cm⁻³) Or graded Ge—GeSn 0 ≦ Sn % ≦ 20% Thicknessrange 100 Å-5000 Å 5 N− Collector ~10000 Å Non-polar GaN (Si-doped ~1 ×10¹⁶ cm⁻³) Wafer Bonded to above 6 N⁺ sub-collector ~5000 Å Non-polarGaN (Si-doped ~5 × 10¹⁸ cm⁻³) 7 Substrate Non-polar GaN N⁺ conductingsubstrate Crystalline

Exemplary advantages of InGaP—GeSn—GaN HBT technology: The NPNInGaP—GeSn—GaN stack minimizes the conduction band offsets, which hinderelectron transport (ultra-fast transistor action). The semiconductormaterials are optimized for performance: GaN collector, GeSn base: InGaPemitter. Wafer bonding allows for the integration of GaN without havingto perform lattice-mismatch growth. Wafer bonding is desirable forGaN—GeSn because the thermal expansion coefficients are close to eachother. Additionally, strain effects can be incorporated in this devicebecause the alloy composition of the InGaP can be changed to introducetensile or compressive strain.

GaN collector for its high saturation velocity and large band gap energywhich results in a high breakdown voltage. In examples described herein,the GaN can be grown on the various substrates like sapphire, SiC, SiGaAs, GaN and template substrates. Polar GaN wurtzite structure can begrown on sapphire, SiC (many polytypes: 3C, 4H, 6H, etc.), Sisubstrates, or template substrates and has piezoelectric andpolarization charge. GaN is grown in the wurtzite (hexagonal) phaseresults in large spontaneous and piezoelectric polarization charge thuspossibly creating a potential energy barrier at the wafer-bonded Ge—GaNbase/collector interface. Non-polar GaN cubic (FCC) structure can begrown on GaAs, Si, or template substrates. GaN in this form has nopolarization charge that degrades the base-collector performance. Acubic form of GaN with (001) orientation can be grown on zinc blendeGaAs. Thus the cubic GaN can be grown on conducting GaAs which can actas the sub-collector. The zinc-blende (cubic) GaN collector has anegligible conduction band offset with respect to the GeSn base. Theconduction band offset between GaAs and cubic GaN is roughlyΔE_(C)˜−0.024 eV. Thus the GeSn (close to Ge) the conduction band offsetto GaN is about ΔE_(C)˜0 eV at the base/collector heterojunction.Additionally, non-polar wurtzite forms, are cut from the c-plane growthalong the “a” or “m” plane directions. If the GaN is grown along the mor a-plane axis, these polarization effects can be eliminated. TypicalGaN wurzite crystals grown along the direction (c-plane) of III-nitridessuffer from polarization induced electric fields. Electric fields do notexist across nonpolar directions (a-plane or m-plane). Thus, highquality non-polar GaN substrate crystals are produced by slicing ac-plane GaN boule along the “a” or “m” plane. This results in low defectdensity non-polar substrates, which have improved substrates forfabrication of devices. Because configuration 5A showed the waferbonding of emitter stack to a polar GaN collector, for thisconfiguration 5B a non-polar GaN substrate is demonstrated. Finally tofully fabricate the device the GaAs top half of HBT stack is waferbonded to the GaN.

The (ordered or disordered) InGaP—GeSn emitter base junction has a largevalence offset (at least 1.1 eV which is much larger than the GeSn bandgap) and a small conduction band offset. This eliminates the backinjection of holes to the emitter from the base, which reduces the gainof the transistor. Also because this is a double HBT, the offset voltagein the output characteristic will be reduced thus enhancing the poweradded efficiency. The base is doped heavily P⁺ (typically>1×10¹⁹ cm⁻³),with such high doping of the base, the emitter valence band offsetblocks the holes even though the base doping is much higher than the N⁻emitter doping (low 10¹⁷ cm⁻³). Furthermore, because GeSn has a lowresistivity of 0.0002 ohm-cm, one can decrease the thickness of the basesignificantly, while still moderately increasing the base sheetresistance value. The frequency response of the device is related to theF_(t) and F_(max). The relationship between transit frequency F_(t) andthe maximum oscillation frequency F_(max) is as follows for an HBT:F_(max)=(F_(t)/8πR_(B)C_(CB))^(1/2). The transit frequency F_(t) isbasically inverse of the time for the electron to traverse the emitter,base and collector. The parameters R_(B) and C_(CB) refer to the basesheet resistance and the capacitance of the collector base junction. Theparameter F_(max) is the unity power gain frequency and indicates themaximum frequency with power gain from a device. The transit frequencycan be further improved by having a higher saturation velocity which theGaN collector exhibits.

In some examples, the key innovation can be the formation of an advancedmanufacturing platform to demonstrate a fully optimized transistorsemiconductor stack, which cannot be grown with standard crystal growthmethodologies. The uniqueness of the device described herein lies in thezero conduction band offset through the three different semiconductormaterials (InGaP—GeSn—GaN) emitter-base-collector optimized for overallHBT performance, which is impossible to grow by standard crystal growthtechniques. The parameters that InGaP—GeSn—GaN NPN transistor canachieve are the following: double heterojunction, emitter-base, and basecollector can reduce offset voltage, high gain (large valence bandoffset at emitter base junction), high breakdown voltages for improvedruggedness for high power applications, and a short collector structurecan result in improved electron transit time.

Exemplary ELO Wafer Bonding Configuration 6A: Fabrication of InGaPEmitter-GeSn Base-GaN Collector double HBT as an example of the ELOwafer bonding device fabrication process. For configuration 6A anepitaxial lift off (ELO) process and wafer bonding can be used tofabricate the emitter/base stack to the GaN collector. Epitaxial liftoff and wafer bonding process is a quick-turn method for integration offabricated GeSn devices to be joined on the GaN substrate. Combining thetechniques of epitaxial lift off and wafer bonding releases therestrictions of lattice matching imposed by epitaxial growth and opensnew degrees of freedom for the design of semiconductor devices, becausethe combination of unique properties of different materials becomespossible.

FIG. 66 shows a schematic methodology of the epitaxial lift off process6600. An epitaxial layer stack top HBT 6608 (preprocessed top half ofthe HBT device: InGaP emitter/GeSn base stack) is grown on a sacrificialGaAs 6606 substrate with a thin aluminum arsenide (AlAs 6607) insertedin between. The top HBT 6608 is covered with wax 6609 for mechanicalstrength, This thin AlAs 6607 sacrificial layer is removed by etching inhydrofluoric acid (HF etch AlAs 6610) in order to lift off the epitaxiallayers from the GaAs 6606 substrate. The wax 6609 protecting the top HBT6608 without the GaAs 6606 substrate is then transferred onto a newsubstrate like GaN 6611 via Van der Waals forces. This technique allowfor the clean and flat surfaces of two dissimilar materials to bebrought into close proximity where attractive forces pull them together,forming an intimate contact between different materials. The strength ofthe adhesion depends on the type of interaction. Van der Waals forcesprovide the first step of attraction. The bonding strength can beincreased in the materials by wafer bonding at elevated temperatures.Both InP and GaAs devices integrated have been fabricated with nearperfect interfaces for bonding to Si, AlN, Sapphire, and LiNbO₃ (apiezoelectric for SAW applications) wafers. FIG. 66 which demonstratesan exemplary ELO process 6600 can be described as follows: (6601)epitaxial HBT stack layer growth with AlAs release layer, top HBT onGaAs with AlAs; (6602) Wax 6609 covers top HBT 6608, epitaxial lift offby HF etch, AlAs 6610 releases the top layer off of GaAs 6606 substrate;(6603) Van der Waals bonding by surface tension of the top HBT 6608 toGaN 6611 substrate; (6604) removal of wax from top HBT 6608 on GaN 6611;and (6605) then wafer bonding to further strengthen the top HBT/GaNmonolithic structure.

Exemplary details of Device Fabrication and growth of ELO top half(example: InGaP—GeSn—GaN HBT). FIG. 67 shows the schematic of the tophalf of the HBT InGaP emitter/GeSn base stack 6701 with the inclusion ofthe AlAs sacrificial layer. Note the sacrificial layer could be AlGaAsfrom 40% to 100% Al. The top half of the HBT InGaP emitter/GeSn basestack 6701 comprises: a sacrificial GaAs substrate 6702; AlAs separationlayer 6703; P⁺ GeSn Base 6704; N⁻ InGaP emitter 6705; N⁺ GaAs contact6706 epitaxial stack. This top half of the HBT InGaP emitter/GeSn basestack 6701 will then be wafer bonded to the GaN collector stack 6707.The GaN collector stack 6707 comprises a starting N⁺ SiC 4H substrate6708, with an N⁺ SiC sub-collector 6709, then finally an N⁻ GaNcollector 6710. There could be many different variations of the GaNcollector stack 6707, such as growth on GaN, Si, GaAs, sapphiresubstrates, or template substrates.

Table 20 shows an exemplary structure top half of the HBT InGaPemitter/GeSn base stack 6701. Table 20 shows an exemplary structure tophalf of the HBT InGaP emitter/GeSn base stack 6701

Layer Layer Name Description Comment 1 N⁺ cap (non-alloyed) ~1,000 ÅInGaAs (Te-doped >10¹⁹ cm⁻³) 2 N− Emitter Cap ~1500 Å GaAs (Si-doped ~5× 10¹⁸ cm⁻³) 3 N− Emitter ~500 Å In_(0.49)Ga_(0.51)P (Si-doped ~3 × 10¹⁷cm⁻³) Ordered or disordered 4 P⁺ Base ~500 Å GeSn (B-doped >10¹⁹ cm⁻³)Or graded Ge—GeSn 0 ≦ Sn % ≦ 20 Thickness range 100 Å-5000 Å 5Sacrificial layer ~50 Å AIAs Release layer 6 High Purity Buffer ~500 ÅGaAs (un-doped) 7 Sacrificial GaAs substrate Semi-insulating orconducting

Table 21 shows an exemplary GaN collector structure 6707. Note that theGaN collector can be grown on Si, SiC, GaAs, Sapphire, and GaN, etc.substrates. Also it is possible to use a SiC collector.

TABLE 21 Exemplary GaN collector structure 6707 Layer Layer NameDescription Comment 1 N⁻ Collector 10,000 Å GaN (Si- Wurtzite phasedoped ~1 × 10¹⁶ cm⁻³) 2 N⁺ sub-collector 5,000 Å 4H SiC Standard(nitrogen (~5 × 10¹⁸ cm⁻³) doped) 3 N⁺ Buffer 500 Å 4H SiC 4 4H SiC(conducting N⁺ substrate: other substrate) phases of SiC possible.

From this point the device to wafer bonded can be pre-processed orpost-processed. For this first exemplary configuration the ELO devicewill be demonstrated pre-processed (device has been partiallyfabricated). The pre-processed top half of HBT is covered in a “blackwax” (Apiezon W) or “white wax” (crystal bond) or other type ofadhesive. In some examples, it is useful to place a mechanical holderlike an exemplary sapphire mechanical substrate to the wax foradditional rigidity and a mechanical strength. The sacrificial AlAslayer is undercut in hydrofluoric HF acid and deionized water at roomtemperature at various ratios. After release the etchant is diluted withde-ionized water, the wax-covered ELO structure is moved to the GaNsubstrate where Van der Waals bonding occurs. In various embodiments,the adhesion process is handled in water to minimize contamination ofthe surfaces.

FIG. 68 shows a pre-processed top half of the HBT which comprises the N⁺GaAs contact 6706, N⁺ InGaP emitter 6705: P⁺ GeSn base 6704; AlAs layer6703 on GaAs substrate 6702; mesa device structure with emitter contact6801 and base contact 6802. Next comes the HF etch of AlAs and ELO 6807step. Before etching the device, an adhesive like wax 6808 is melted onpre-processed fabricated Top Half of HBT 6800 and sometimes it can beuseful to have a mechanical substrate 6810 place on the wax foradditional mechanical strength. This can be useful in large waferdevices (2″, 3″, 4″, 6″, 12″, 18″, etc., wafer sizes and not onlylimited to these sizes). For the ELO process the wax 6808 coatedpre-processed fabricated Top Half of HBT 6801 assembly is placed into asolution of anhydrous hydrofluoric (HF) acid and over time the waxcoated top half of the HBT 6809 is removed from the sacrificial GaAssubstrate 6702. The solution can be heated and temperature controlled,the lift off process time depends on area of the device, and may takefrom several minutes to many hours. Once the sample is lift-off, the waxprovides for mechanical strength of the lifted off layer and allows forease of transport to the GaN substrate.

FIG. 69 shows where the top half of HBT wafer bonded to collectorstructure 6901, which comprises the wax coated top of the HBT 6809placed on the GaN collector structure 6707 with van der Waals bonding.Note the GaN collector could have a layer to electrically enhanceperformance, which can be deposited by PLD any other epitaxial process.There are many other seed layers that could be used for this purpose.The seed layer is could be used for adhesion or modifying the electricalinterface properties of the heterojunction to improve performance orreliability.

Initially, the wax coated top of the HBT 6809 placed on the GaNcollector structure 6707 with van der Waals bonding results in theadhesion of layers. The structure is put into trichloroethylene oracetone or some solvent to remove the wax, which then forms the waferbonded HBT 6902. To finalize the device for test a bottom metal contact6903 is applied to the N⁺ SiC 4H substrate 6708. The final wafer bondedstructure may then be placed in a wafer bonder, and under heat andpressure, stronger bond formation between the top half of the HBT andthe GaN collector structure 6707 should result for a permanent finalstructure. Finally, bottom metallization of the structure allows for thetesting of this heterojunction bipolar structure for DC testing in astandard emitter-base-collector configuration. For RF testing contactscan be in this case put on top of the sub-collector to reduce thecapacitance effects of the substrate, but this uses standard RF devicefabrication techniques.

Exemplary Inverted Wafer Bonding Configuration 6B: Device Fabrication &Growth of Inverted Top Half of the GeSn Base HBT for Wafer Bonding andPost Processing

Additionally, it can be useful to use an inverted top of the HBT forwafer bonding. FIG. 70 shows an inverted top half of HBT 7000. Thiscomprises growth on a sacrificial GaAs substrate 7001; followed by alatticed matched InGaP etch stop 7002, an N⁺ contact GaAs 7003, then aN⁻ GaAs emitter 7004, and then the P⁺ GeSn base 7005. Table 22 show anexemplary design of the structure.

TABLE 22 Exemplary epitaxial structure of the inverted top half of HBTLayer Layer Name Description Comment 1 P⁺ Base ~500 Å GeSn (B-doped>10¹⁹ cm⁻³) Or graded Ge—GeSn 0 ≦ Sn % ≦ 20 Thickness range 100 Å-5000 Å2 N⁻ Emitter ~500 Å In_(0.49)Ga_(0.51)P (Si-doped ~3 × 10¹⁷ cm⁻³)Ordered or disordered 3 N⁻ Emitter Cap ~1500 Å GaAs (Si-doped ~5 × 10¹⁸cm⁻³) 4 N⁺ Contact ~1000 Å GaAs (Si-doped ~5 × 10¹⁸ cm⁻³) 5 ~50 Å InGaPStop Etch 6 High Purity Buffer ~500 Å GaAs UID 7 GaAs substrateSemi-insulating or conducting

FIG. 71 shows the straightforward wafer bonding 7101 of the inverted tophalf of HBT 7000 to the GaN collector structure 6707. This step showsthe final wafer bonded structure and removal of the sacrificial GaAssubstrate and InGaP stop etch 7102. The wafer bonded junction occurs atthe P⁺ GeSn base to the N− GaN collector. The wafer bonding processwould occur under heat, pressure, time, current/voltage bias, gaseousenvironment, etc. The final step is the wafer bonded structure andremoval of sacrificial GaAs substrate 7001 and InGaP Stop etch 7002.Thus, after wafer bonding, the sacrificial GaAs substrate 7001 and InGaPetch stop 7002 would be removed by lapping (thinning) and then etchingto the InGaP etch stop layer. This leaves the final epitaxial waferbonded HBT structure 7103.

FIG. 72 shows the post-processing of final epitaxial wafer bonded HBTstructure 7103, where a standard quick lot HBT device fabricationprocedure can be used to test the HBT. The device fabrication of thestructure relies on standard GaAs fabrication techniques since the waferbonded junction 7204 never gets exposed to any of the chemicalprocesses. This standard device processing technology is used for thefabrication of HBTs and can be generally used for all the differentconfigurations previously elucidated. This HBT test mask set willprovide immediate feedback for fine tuning the growth process HBT devicefabricated using a Quick lot mask set. There are many methodologies forthe fabrication of the HBT. One possible version is to apply photoresistacross the surface of the HBT structure 7103. A photomask will be placedon the photoresist surface and exposed with UV light which imprints apattern of the emitter metal contact 7201 across the surface. Next, theHBT structure 7103 with exposed photoresist is put into a developersolution. The pattern is developed and then metal is blanket coatedacross the surface using a metal evaporator. The whole structure is putinto acetone which dissolves the photoresist leaving only emitter metalcontact 7201 pattern across the top of the N⁺ GaAs contact 7003.

The emitter metal contact 7201 than can act as a metal mask for mesaetching the HBT structure 7103 down to the P⁺ GeSn Base 7005. Here,photoresist is spun all over the mesa etched structure. With a photomaskthat has base metal contact 7202 pattern is placed on the photoresist.The photomask covering the photoresist is subsequently exposed with UVlight and then developed to open a pattern where the base metal contact7202 can be deposited on the P⁺ GeSn base 7005. Typically a metalevaporator will deposit blanket metal all over the surface of the mesaetched structure. The structure will then be put in acetone for metallift off, thus resulting in a pattern of base metal contact 7202 on theP⁺ GeSn Base 7005.

Next, back metallization or the bottom metal contact 7203 is applied tothe N⁺ SiC 4H substrate 6708. FIG. 72 shows the final wafer bonded HBTstructure 7200. Sometimes the HBT device needs to be alloyed at elevatedtemperature to activate the metal contacts. The device is ready for DCtesting.

Typical parameters that are measured and used to qualify the HBTmaterials are sheet resistance of the emitter, base and sub-collector,by both TLM and van Der Pauw cross structures. Various sized HBTs(emitter sizes are 40×40, 50×50, 75×75, 100×100 μm²) are used todetermine effects of geometry to device parameters such as Gummel, Gain,Output Characteristics and breakdown voltages.

To summarize, the devices are fabricated using standard semiconductorprocess techniques. For the PN junction fabrication a single mask levelfor the etching of the base and ohmic anneals will be used. The processwill use mesa wet-etch and metallization lift-off techniques common inHBT fabrication. AuGeNiAu or other metals can be used for the N-typeGaAs materials and Al to P-type GeSn. The junctions of interest are theemitter-base junction, and the base-collector junction.

Exemplary Embodiment

Base region with all the above compositional Ge—GeSn grading variationsof the base from emitter side to collector side.

Exemplary Embodiment

Base region including all the variations and inclusion of a GeSn quantumwell or GeSn quantum dot structure in the base region making a lightemitting transistor laser.

Exemplary Summary of HBT parameters: The embodiments described hereincan relate to the following: any bipolar transistor using a GeSn base;any bipolar transistor using a compositionally graded Ge—GeSn baseand/or any light emitting bipolar transistor laser using a GeSn quantumwell or quantum dot in the base region.

It should be noted that the values of the bandgap energies can changedue to growth conditions and other factors. The values of the conductionand valence band offsets between dissimilar heterojunctionsemiconductors are used as guidelines and can be different dependent onthe growth conditions, doping levels, and other factors. Theseparameters are also dependent on the temperature of the materials.

Although the embodiments have been described with reference to specificembodiments, it will be understood by those skilled in the art thatvarious changes may be made without departing from the spirit or scopeof the invention. Accordingly, the disclosure of embodiments of theinvention is intended to be illustrative of the scope of the inventionand is not intended to be limiting. It is intended that the scope of theinvention shall be limited only to the extent required by the appendedclaims. For example, to one of ordinary skill in the art, it will bereadily apparent that the methods, processes, and activities describedherein may be comprised of many different activities, procedures and beperformed by many different modules, in many different orders that anyelement of the figures may be modified and that the foregoing discussionof certain of these embodiments does not necessarily represent acomplete description of all possible embodiments.

All elements claimed in any particular claim are essential to theembodiment claimed in that particular claim. Consequently, replacementof one or more claimed elements constitutes reconstruction and notrepair. Additionally, benefits, other advantages, and solutions toproblems have been described with regard to specific embodiments. Thebenefits, advantages, solutions to problems, and any element or elementsthat may cause any benefit, advantage, or solution to occur or becomemore pronounced, however, are not to be construed as critical, required,or essential features or elements of any or all of the claims, unlesssuch benefits, advantages, solutions, or elements are stated in suchclaim.

Moreover, embodiments and limitations disclosed herein are not dedicatedto the public under the doctrine of dedication if the embodiments and/orlimitations: (1) are not expressly claimed in the claims; and (2) are orare potentially equivalents of express elements and/or limitations inthe claims under the doctrine of equivalents.

1. A heterojunction bipolar transistor comprising: a GeSn base region.2. The heterojunction bipolar transistor of claim 1, wherein theheterojunction bipolar transistor is an NPN device.
 3. Theheterojunction bipolar transistor of claim 1, wherein the GeSn baseregion comprises a graded Ge—GeSn base region.
 4. The heterojunctionbipolar transistor of claim 1, wherein the GeSn base region comprises aSn content varying from 0% to approximately 20% or less.
 5. Theheterojunction bipolar transistor of claim 1, wherein the GeSn baseregion comprises a GeSn quantum well or a GeSn quantum dot, and whereinthe GeSn quantum well or dot is part of an active region of a lightemitting transistor laser.
 6. The heterojunction bipolar transistor ofclaim 1, wherein: a first conduction band offset of an emitter basejunction of the heterojunction bipolar transistor is approximately zero;and a second conduction band offset of a base collector junction of theheterojunction bipolar transistor is approximately zero.
 7. Theheterojunction bipolar transistor of claim 1, further comprising: anemitter comprising GaAs; and a collector comprising GaAs.
 8. Theheterojunction bipolar transistor of claim 1, further comprising: anemitter comprising InGaP; and a collector comprising GaAs.
 9. Theheterojunction bipolar transistor of claim 9, wherein the InGaP isordered.
 10. The heterojunction bipolar transistor of claim 1, furthercomprising: an emitter comprising AlGaAs; and a collector comprisingGaAs.
 11. The heterojunction bipolar transistor of claim 1, furthercomprising: an emitter comprising GaAs; and a collector comprising GaN.12. The heterojunction bipolar transistor of claim 11, wherein thecollector is non-polar.
 13. The heterojunction bipolar transistor ofclaim 1, further comprising: an emitter comprising InGaP; and acollector comprising GaN.
 14. The heterojunction bipolar transistor ofclaim 13, wherein the InGaP is ordered.
 15. The heterojunction bipolartransistor of claim 13, wherein: the GaN is non-polar.
 16. Theheterojunction bipolar transistor of claim 1, further comprising: anemitter comprising SiGeSn; and a collector comprising SiGeSn.
 17. Theheterojunction bipolar transistor of claim 16, wherein: the SiGeSn ofthe emitter is lattice matched to the GeSn base region; and the SiGeSnof the collector is lattice matched to the GeSn base region.
 18. Theheterojunction bipolar transistor of claim 16, wherein: the SiGeSn ofthe emitter strains the GeSn base region; and the SiGeSn of thecollector strains the GeSn base region.
 19. The heterojunction bipolartransistor of claim 18, wherein the strain is biaxial tensile stress.20. The heterojunction bipolar transistor of claim 18, wherein thestrain is compressive stress.
 21. The heterojunction bipolar transistorof claim 1, wherein the GeSn base region is located over a Si substrate.22. The heterojunction bipolar transistor of claim 1, wherein theheterojunction bipolar transistor comprises a laser.
 23. Theheterojunction bipolar transistor of claim 1, wherein the GeSn baseregion comprises compressively strained GeSn.
 24. The heterojunctionbipolar transistor of claim 1, wherein the GeSn base region comprisesrelaxed GeSn.
 25. The heterojunction bipolar transistor of claim 1,wherein the GeSn base region comprises GeSn as an indirect bandgapsemiconductor.
 26. The heterojunction bipolar transistor of claim 1,wherein the GeSn base region comprises GeSn as a direct bandgapsemiconductor.
 27. The heterojunction bipolar transistor of claim 1,wherein the GeSn base region comprises a GeSn active region within alight emitting device or a transistor laser.
 28. The heterojunctionbipolar transistor of claim 1, wherein the GeSn base region comprises adirect bandgap GeSn active region that is part of a light emittingdevice or transistor laser.
 29. A method of manufacturing aheterojunction bipolar transistor comprising: forming a GeSn baseregion.
 30. The method of claim 29, wherein forming the GeSn base regioncomprises forming the GeSn base region to comprise a GeSn quantum wellor GeSn quantum dot, wherein the GeSn quantum well or dot is surroundedby GaAs or Ge P-type regions.
 31. The method of claim 29, furthercomprising wafer bonding a GaN collector to the GeSn base region.
 32. Adevice comprising: a first heterojunction bipolar transistor comprisinga PNP device having a first GeSn base; and a second heterojunctionbipolar transistor comprising an NPN device having a second GeSn base,wherein the first and second heterojunction bipolar transistors arelocated over a common substrate.
 33. A method of manufacturing a devicecomprising: forming a first heterojunction bipolar transistor comprisinga PNP device having a first GeSn base; and forming a secondheterojunction bipolar transistor comprising an NPN device having asecond GeSn base, wherein forming the first and second heterojunctionbipolar transistors occur simultaneously with each other over a commonsubstrate.
 34. A device comprising: a first bipolar transistorcomprising a first GeSn base; and a second bipolar transistor comprisinga second GeSn base, wherein the first and second bipolar transistors arecomplementary devices and are located over a common substrate.
 35. Amethod of manufacturing a device comprising: forming a first bipolartransistor comprising a first GeSn base; and forming a second bipolartransistor comprising a second GeSn base, wherein forming the first andsecond bipolar transistors occur simultaneously with each other over acommon substrate.
 36. A bipolar transistor comprising: a GeSn baseregion.
 37. A method of manufacturing a bipolar transistor, the methodcomprising: providing a GeSn base region.
 38. A transistor lasercomprising: a GeSn active region.
 39. The transistor laser of claim 38,wherein the GeSn active region comprises a GeSn quantum well.
 40. Thetransistor laser of claim 39, wherein the GeSn quantum well comprises aSn percentage of approximately 0% to 10% within the GeSn quantum well.41. The transistor laser of claim 38, wherein the GeSn active regioncomprises a GeSn quantum dot.
 42. The transistor laser of claim 41,wherein the GeSn quantum dot comprises a Sn percentage of approximately10% to 20% within the GeSn quantum dot.
 43. The transistor laser ofclaim 38, wherein the GeSn active region comprises a GeSn quantum wellor a GeSn quantum dot, and wherein the GeSn quantum well or dot islocated within a Ge barrier.
 44. The transistor laser of claim 38,wherein the GeSn active region comprises a GeSn quantum well or a GeSnquantum dot, and wherein the GeSn quantum well or dot is located withina GaAs barrier.
 45. The transistor laser of claim 38, wherein the GeSnactive region comprises a GeSn quantum well or a GeSn quantum dot, andwherein a laser structure for the transistor laser is identical for theGeSn quantum well and GeSn quantum dot.
 46. A method of forming atransistor laser, the method comprising: forming a GeSn active region.